diff --git a/src/db/db/dbCircuit.cc b/src/db/db/dbCircuit.cc
index a2d5036bb..0c80b380a 100644
--- a/src/db/db/dbCircuit.cc
+++ b/src/db/db/dbCircuit.cc
@@ -333,20 +333,40 @@ void Circuit::remove_pin (size_t id)
 
 void Circuit::add_net (Net *net)
 {
+  if (! net) {
+    return;
+  }
+  if (net->circuit ()) {
+    throw tl::Exception (tl::to_string (tr ("Net already part of a circuit")));
+  }
+
   m_nets.push_back (net);
   net->set_circuit (this);
 }
 
 void Circuit::remove_net (Net *net)
 {
+  if (! net) {
+    return;
+  }
+  if (net->circuit () != this) {
+    throw tl::Exception (tl::to_string (tr ("Net not withing given circuit")));
+  }
+
   m_nets.erase (net);
 }
 
 void Circuit::join_nets (Net *net, Net *with)
 {
+  if (! net) {
+    return;
+  }
   if (net == with || ! with) {
     return;
   }
+  if (net->circuit () != this || with->circuit () != this) {
+    throw tl::Exception (tl::to_string (tr ("Nets not withing given circuit")));
+  }
 
   while (with->begin_terminals () != with->end_terminals ()) {
     db::Device *device = const_cast<db::Device *> (with->begin_terminals ()->device ());
@@ -372,6 +392,13 @@ void Circuit::join_nets (Net *net, Net *with)
 
 void Circuit::add_device (Device *device)
 {
+  if (! device) {
+    return;
+  }
+  if (device->circuit ()) {
+    throw tl::Exception (tl::to_string (tr ("Device already in a circuit")));
+  }
+
   device->set_circuit (this);
 
   size_t id = 0;
@@ -386,11 +413,25 @@ void Circuit::add_device (Device *device)
 
 void Circuit::remove_device (Device *device)
 {
+  if (! device) {
+    return;
+  }
+  if (device->circuit () != this) {
+    throw tl::Exception (tl::to_string (tr ("Device not withing given circuit")));
+  }
+
   m_devices.erase (device);
 }
 
 void Circuit::add_subcircuit (SubCircuit *subcircuit)
 {
+  if (! subcircuit) {
+    return;
+  }
+  if (subcircuit->circuit ()) {
+    throw tl::Exception (tl::to_string (tr ("Subcircuit already in a circuit")));
+  }
+
   subcircuit->set_circuit (this);
 
   size_t id = 0;
@@ -405,6 +446,13 @@ void Circuit::add_subcircuit (SubCircuit *subcircuit)
 
 void Circuit::remove_subcircuit (SubCircuit *subcircuit)
 {
+  if (! subcircuit) {
+    return;
+  }
+  if (subcircuit->circuit () != this) {
+    throw tl::Exception (tl::to_string (tr ("Subcircuit not withing given circuit")));
+  }
+
   m_subcircuits.erase (subcircuit);
 }
 
@@ -420,7 +468,12 @@ void Circuit::unregister_ref (SubCircuit *r)
 
 void Circuit::flatten_subcircuit (SubCircuit *subcircuit)
 {
-  tl_assert (subcircuit != 0);
+  if (! subcircuit) {
+    return;
+  }
+  if (subcircuit->circuit () != this) {
+    throw tl::Exception (tl::to_string (tr ("Subcircuit not withing given circuit")));
+  }
 
   const db::Circuit *c = subcircuit->circuit_ref ();
 
diff --git a/src/db/db/dbNetlist.cc b/src/db/db/dbNetlist.cc
index bd33399d5..dd32a00ab 100644
--- a/src/db/db/dbNetlist.cc
+++ b/src/db/db/dbNetlist.cc
@@ -265,6 +265,10 @@ void Netlist::unlock ()
 
 const tl::vector<Circuit *> &Netlist::child_circuits (Circuit *circuit)
 {
+  if (circuit->netlist () != this) {
+    throw tl::Exception (tl::to_string (tr ("Circuit not within given netlist")));
+  }
+
   if (! m_valid_topology) {
     validate_topology ();
   }
@@ -275,6 +279,10 @@ const tl::vector<Circuit *> &Netlist::child_circuits (Circuit *circuit)
 
 const tl::vector<Circuit *> &Netlist::parent_circuits (Circuit *circuit)
 {
+  if (circuit->netlist () != this) {
+    throw tl::Exception (tl::to_string (tr ("Circuit not within given netlist")));
+  }
+
   if (! m_valid_topology) {
     validate_topology ();
   }
@@ -364,18 +372,39 @@ void Netlist::clear ()
 
 void Netlist::add_circuit (Circuit *circuit)
 {
+  if (! circuit) {
+    return;
+  }
+  if (circuit->netlist ()) {
+    throw tl::Exception (tl::to_string (tr ("Circuit already contained in a netlist")));
+  }
+
   m_circuits.push_back (circuit);
   circuit->set_netlist (this);
 }
 
 void Netlist::remove_circuit (Circuit *circuit)
 {
+  if (! circuit) {
+    return;
+  }
+  if (circuit->netlist () != this) {
+    throw tl::Exception (tl::to_string (tr ("Circuit not within given netlist")));
+  }
+
   circuit->set_netlist (0);
   m_circuits.erase (circuit);
 }
 
 void Netlist::purge_circuit (Circuit *circuit)
 {
+  if (! circuit) {
+    return;
+  }
+  if (circuit->netlist () != this) {
+    throw tl::Exception (tl::to_string (tr ("Circuit not within given netlist")));
+  }
+
   circuit->blank ();
   remove_circuit (circuit);
 }
@@ -406,7 +435,12 @@ void Netlist::flatten_circuits (const std::vector<Circuit *> &circuits)
 
 void Netlist::flatten_circuit (Circuit *circuit)
 {
-  tl_assert (circuit != 0);
+  if (! circuit) {
+    return;
+  }
+  if (circuit->netlist () != this) {
+    throw tl::Exception (tl::to_string (tr ("Circuit not within given netlist")));
+  }
 
   std::vector<db::SubCircuit *> refs;
   for (db::Circuit::refs_iterator sc = circuit->begin_refs (); sc != circuit->end_refs (); ++sc) {
@@ -448,24 +482,52 @@ DeviceClass *Netlist::device_class_by_name (const std::string &name)
 
 void Netlist::add_device_class (DeviceClass *device_class)
 {
+  if (! device_class) {
+    return;
+  }
+  if (device_class->netlist ()) {
+    throw tl::Exception (tl::to_string (tr ("Device class already contained in a netlist")));
+  }
+
   m_device_classes.push_back (device_class);
   device_class->set_netlist (this);
 }
 
 void Netlist::remove_device_class (DeviceClass *device_class)
 {
+  if (! device_class) {
+    return;
+  }
+  if (device_class->netlist () != this) {
+    throw tl::Exception (tl::to_string (tr ("Device class not within given netlist")));
+  }
+
   device_class->set_netlist (0);
   m_device_classes.erase (device_class);
 }
 
 void Netlist::add_device_abstract (DeviceAbstract *device_abstract)
 {
+  if (! device_abstract) {
+    return;
+  }
+  if (device_abstract->netlist ()) {
+    throw tl::Exception (tl::to_string (tr ("Device abstract already contained in a netlist")));
+  }
+
   m_device_abstracts.push_back (device_abstract);
   device_abstract->set_netlist (this);
 }
 
 void Netlist::remove_device_abstract (DeviceAbstract *device_abstract)
 {
+  if (! device_abstract) {
+    return;
+  }
+  if (device_abstract->netlist () != this) {
+    throw tl::Exception (tl::to_string (tr ("Device abstract not within given netlist")));
+  }
+
   device_abstract->set_netlist (0);
   m_device_abstracts.erase (device_abstract);
 }
diff --git a/testdata/ruby/dbNetlist.rb b/testdata/ruby/dbNetlist.rb
index 3fc2e64ce..c927e9dfb 100644
--- a/testdata/ruby/dbNetlist.rb
+++ b/testdata/ruby/dbNetlist.rb
@@ -68,9 +68,21 @@ class DBNetlist_TestClass < TestBase
     cc.dont_purge = true
     assert_equal(cc.dont_purge, true)
 
+    begin
+      nl.remove(cc) # not in netlist yet
+      assert_equal(false, true)
+    rescue
+    end
+
     nl.add(cc)
     cc.name = "ABC"
 
+    begin
+      nl.add(cc) # already in netlist
+      assert_equal(false, true)
+    rescue
+    end
+
     names = []
     nl.each_circuit { |i| names << i.name }
     assert_equal(names, [ c.name, cc.name ])
@@ -119,9 +131,22 @@ class DBNetlist_TestClass < TestBase
     assert_equal(c.name, "XYZ")
 
     cc = RBA::GenericDeviceClass::new
+
+    begin
+      nl.remove(cc) # not in netlist yet
+      assert_equal(false, true)
+    rescue
+    end
+
     nl.add(cc)
     cc.name = "ABC"
 
+    begin
+      nl.add(cc) # already in netlist
+      assert_equal(false, true)
+    rescue
+    end
+
     names = []
     nl.each_device_class { |i| names << i.name }
     assert_equal(names, [ c.name, cc.name ])