diff --git a/src/lay/lay/doc/manual/lvs_tweaks.xml b/src/lay/lay/doc/manual/lvs_tweaks.xml
index d892afb3e..9e9e81dd1 100644
--- a/src/lay/lay/doc/manual/lvs_tweaks.xml
+++ b/src/lay/lay/doc/manual/lvs_tweaks.xml
@@ -105,7 +105,7 @@
   </p>
 
   <p>
-  To flatten a circuit from the extracted netlist use
+  To flatten a circuit from the extracted netlist use <class_doc href="Netlist#flatten_circuit"/>:
   </p>
 
   <pre>netlist.flatten_circuit("CIRCUIT_NAME")</pre>
diff --git a/src/laybasic/laybasic/layIndexedNetlistModel.h b/src/laybasic/laybasic/layIndexedNetlistModel.h
index 129eaf561..422a67a58 100644
--- a/src/laybasic/laybasic/layIndexedNetlistModel.h
+++ b/src/laybasic/laybasic/layIndexedNetlistModel.h
@@ -30,6 +30,7 @@
 #include <map>
 #include <vector>
 #include <algorithm>
+#include <limits>
 
 namespace db
 {
@@ -48,6 +49,8 @@ namespace db
 namespace lay
 {
 
+const size_t no_netlist_index = std::numeric_limits<size_t>::max ();
+
 /**
  *  @brief An interface to supply the netlist browser model with indexed items
  */
diff --git a/src/laybasic/laybasic/layNetlistBrowserModel.cc b/src/laybasic/laybasic/layNetlistBrowserModel.cc
index 2ce92ea97..b47a460a1 100644
--- a/src/laybasic/laybasic/layNetlistBrowserModel.cc
+++ b/src/laybasic/laybasic/layNetlistBrowserModel.cc
@@ -240,89 +240,141 @@ NetlistBrowserModel::~NetlistBrowserModel ()
 void *
 NetlistBrowserModel::make_id_circuit (size_t circuit_index) const
 {
-  return make_id (circuit_index);
+  if (circuit_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    return make_id (circuit_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_pin (size_t circuit_index, size_t pin_index) const
 {
-  return make_id (circuit_index, mp_indexer->circuit_count (), 1, 8, pin_index);
+  if (circuit_index == lay::no_netlist_index || pin_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    return make_id (circuit_index, mp_indexer->circuit_count (), 1, 8, pin_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_pin_net (size_t circuit_index, size_t pin_index, size_t net_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 1, 8, pin_index, mp_indexer->pin_count (circuits), net_index + 1);
+  if (circuit_index == lay::no_netlist_index || pin_index == lay::no_netlist_index || net_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 1, 8, pin_index, mp_indexer->pin_count (circuits), net_index + 1);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_net (size_t circuit_index, size_t net_index) const
 {
-  return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index);
+  if (circuit_index == lay::no_netlist_index || net_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_net_device_terminal (size_t circuit_index, size_t net_index, size_t terminal_ref_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 1, 4, terminal_ref_index);
+  if (circuit_index == lay::no_netlist_index || net_index == lay::no_netlist_index || terminal_ref_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 1, 4, terminal_ref_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_net_device_terminal_others (size_t circuit_index, size_t net_index, size_t terminal_ref_index, size_t other_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  IndexedNetlistModel::net_pair nets = nets_from_id (make_id_circuit_net (circuit_index, net_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 1, 4, terminal_ref_index, mp_indexer->net_terminal_count (nets), other_index + 1);
+  if (circuit_index == lay::no_netlist_index || net_index == lay::no_netlist_index || terminal_ref_index == lay::no_netlist_index || other_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    IndexedNetlistModel::net_pair nets = nets_from_id (make_id_circuit_net (circuit_index, net_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 1, 4, terminal_ref_index, mp_indexer->net_terminal_count (nets), other_index + 1);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_net_pin (size_t circuit_index, size_t net_index, size_t pin_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 2, 4, pin_index);
+  if (circuit_index == lay::no_netlist_index || net_index == lay::no_netlist_index || pin_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 2, 4, pin_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_net_subcircuit_pin (size_t circuit_index, size_t net_index, size_t pin_ref_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 3, 4, pin_ref_index);
+  if (circuit_index == lay::no_netlist_index || net_index == lay::no_netlist_index || pin_ref_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 3, 4, pin_ref_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_net_subcircuit_pin_others (size_t circuit_index, size_t net_index, size_t pin_ref_index, size_t other_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  IndexedNetlistModel::net_pair nets = nets_from_id (make_id_circuit_net (circuit_index, net_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 3, 4, pin_ref_index, mp_indexer->net_subcircuit_pin_count (nets), other_index + 1);
+  if (circuit_index == lay::no_netlist_index || net_index == lay::no_netlist_index || pin_ref_index == lay::no_netlist_index || other_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    IndexedNetlistModel::net_pair nets = nets_from_id (make_id_circuit_net (circuit_index, net_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 2, 8, net_index, mp_indexer->net_count (circuits), 3, 4, pin_ref_index, mp_indexer->net_subcircuit_pin_count (nets), other_index + 1);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_subcircuit (size_t circuit_index, size_t subcircuit_index) const
 {
-  return make_id (circuit_index, mp_indexer->circuit_count (), 3, 8, subcircuit_index);
+  if (circuit_index == lay::no_netlist_index || subcircuit_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    return make_id (circuit_index, mp_indexer->circuit_count (), 3, 8, subcircuit_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_subcircuit_pin (size_t circuit_index, size_t subcircuit_index, size_t pin_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 3, 8, subcircuit_index, mp_indexer->subcircuit_count (circuits), pin_index + 1);
+  if (circuit_index == lay::no_netlist_index || subcircuit_index == lay::no_netlist_index || pin_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 3, 8, subcircuit_index, mp_indexer->subcircuit_count (circuits), pin_index + 1);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_device (size_t circuit_index, size_t device_index) const
 {
-  return make_id (circuit_index, mp_indexer->circuit_count (), 4, 8, device_index);
+  if (circuit_index == lay::no_netlist_index || device_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    return make_id (circuit_index, mp_indexer->circuit_count (), 4, 8, device_index);
+  }
 }
 
 void *
 NetlistBrowserModel::make_id_circuit_device_terminal (size_t circuit_index, size_t device_index, size_t terminal_index) const
 {
-  IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
-  return make_id (circuit_index, mp_indexer->circuit_count (), 4, 8, device_index, mp_indexer->device_count (circuits), terminal_index + 1);
+  if (circuit_index == lay::no_netlist_index || device_index == lay::no_netlist_index || terminal_index == lay::no_netlist_index) {
+    return no_id;
+  } else {
+    IndexedNetlistModel::circuit_pair circuits = circuits_from_id (make_id_circuit (circuit_index));
+    return make_id (circuit_index, mp_indexer->circuit_count (), 4, 8, device_index, mp_indexer->device_count (circuits), terminal_index + 1);
+  }
 }
 
 bool
@@ -2238,7 +2290,11 @@ NetlistBrowserModel::subcircuit_from_index (const QModelIndex &index) const
 QModelIndex
 NetlistBrowserModel::index_from_id (void *id, int column) const
 {
-  if (is_id_circuit (id)) {
+  if (id == no_id) {
+
+    return QModelIndex ();
+
+  } else if (is_id_circuit (id)) {
 
     return createIndex (circuit_index_from_id (id), column, id);
 
diff --git a/src/laybasic/laybasic/layNetlistCrossReferenceModel.cc b/src/laybasic/laybasic/layNetlistCrossReferenceModel.cc
index 1e7eda47d..f71465227 100644
--- a/src/laybasic/laybasic/layNetlistCrossReferenceModel.cc
+++ b/src/laybasic/laybasic/layNetlistCrossReferenceModel.cc
@@ -393,7 +393,9 @@ size_t NetlistCrossReferenceModel::circuit_index (const circuit_pair &circuits)
     }
 
     i = m_index_of_circuits.find (circuits);
-    tl_assert (i != m_index_of_circuits.end ());
+    if (i == m_index_of_circuits.end ()) {
+      return lay::no_netlist_index;
+    }
 
   }
 
@@ -403,35 +405,50 @@ size_t NetlistCrossReferenceModel::circuit_index (const circuit_pair &circuits)
 size_t NetlistCrossReferenceModel::net_index (const net_pair &nets) const
 {
   circuit_pair circuits = parent_of (nets);
-  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
+
   const db::NetlistCrossReference::PerCircuitData *org_data = mp_cross_ref->per_circuit_data_for (circuits);
-  tl_assert (org_data != 0);
+  if (! org_data) {
+    return lay::no_netlist_index;
+  }
+
+  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
   return get_index_of (nets, org_data->nets.begin (), org_data->nets.end (), data->index_of_nets);
 }
 
 size_t NetlistCrossReferenceModel::device_index (const device_pair &devices) const
 {
   circuit_pair circuits = parent_of (devices);
-  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
+
   const db::NetlistCrossReference::PerCircuitData *org_data = mp_cross_ref->per_circuit_data_for (circuits);
-  tl_assert (org_data != 0);
+  if (! org_data) {
+    return lay::no_netlist_index;
+  }
+
+  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
   return get_index_of (devices, org_data->devices.begin (), org_data->devices.end (), data->index_of_devices);
 }
 
 size_t NetlistCrossReferenceModel::pin_index (const pin_pair &pins, const circuit_pair &circuits) const
 {
-  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
   const db::NetlistCrossReference::PerCircuitData *org_data = mp_cross_ref->per_circuit_data_for (circuits);
-  tl_assert (org_data != 0);
+  if (! org_data) {
+    return lay::no_netlist_index;
+  }
+
+  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
   return get_index_of (pins, org_data->pins.begin (), org_data->pins.end (), data->index_of_pins);
 }
 
 size_t NetlistCrossReferenceModel::subcircuit_index (const subcircuit_pair &subcircuits) const
 {
   circuit_pair circuits = parent_of (subcircuits);
-  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
+
   const db::NetlistCrossReference::PerCircuitData *org_data = mp_cross_ref->per_circuit_data_for (circuits);
-  tl_assert (org_data != 0);
+  if (! org_data) {
+    return lay::no_netlist_index;
+  }
+
+  PerCircuitCacheData *data = & m_per_circuit_data [circuits];
   return get_index_of (subcircuits, org_data->subcircuits.begin (), org_data->subcircuits.end (), data->index_of_subcircuits);
 }