diff --git a/network/ConcreteNetwork.cc b/network/ConcreteNetwork.cc
index 9586d2d1..46973913 100644
--- a/network/ConcreteNetwork.cc
+++ b/network/ConcreteNetwork.cc
@@ -1141,14 +1141,20 @@ ConcreteNetwork::makePins(Instance *inst)
 void
 ConcreteNetwork::replaceCell(Instance *inst,
 			     LibertyCell *cell)
+{
+  replaceCellIntenal(inst, cell);
+}
+
+void
+ConcreteNetwork::replaceCellIntenal(Instance *inst,
+				    ConcreteCell *cell)
 {
   InstancePinIterator *pin_iter = pinIterator(inst);
-  ConcreteCell *ccell = cell;
   while (pin_iter->hasNext()) {
     Pin *pin = pin_iter->next();
     ConcretePin *cpin = reinterpret_cast<ConcretePin*>(pin);
     ConcretePort *pin_cport = reinterpret_cast<ConcretePort*>(cpin->port());
-    ConcretePort *cport = ccell->findPort(pin_cport->name());
+    ConcretePort *cport = cell->findPort(pin_cport->name());
     if (cport)
       cpin->port_ = cport;
     else
@@ -1611,7 +1617,7 @@ ConcreteInstance::deleteNet(ConcreteNet *net)
 }
 
 void
-ConcreteInstance::setCell(LibertyCell *cell)
+ConcreteInstance::setCell(ConcreteCell *cell)
 {
   cell_ = cell;
 }
diff --git a/network/ConcreteNetwork.hh b/network/ConcreteNetwork.hh
index 5b1109e3..d43b66ec 100644
--- a/network/ConcreteNetwork.hh
+++ b/network/ConcreteNetwork.hh
@@ -247,6 +247,8 @@ protected:
 			ConcretePin *cpin);
   void connectNetPin(ConcreteNet *cnet,
 		     ConcretePin *cpin);
+  void replaceCellIntenal(Instance *inst,
+			  ConcreteCell *cell);
 
   // Cell lookup search order sequence.
   ConcreteLibrarySeq library_seq_;
@@ -284,7 +286,7 @@ public:
   void addNet(const char *name,
 	      ConcreteNet *net);
   void deleteNet(ConcreteNet *net);
-  void setCell(LibertyCell *cell);
+  void setCell(ConcreteCell *cell);
   void initPins();
 
 protected:
diff --git a/tcl/NetworkEdit.tcl b/tcl/NetworkEdit.tcl
index 1afb1ecd..8174560e 100644
--- a/tcl/NetworkEdit.tcl
+++ b/tcl/NetworkEdit.tcl
@@ -254,7 +254,10 @@ proc insert_buffer { buffer_name buffer_cell net load_pins buffer_out_net_name }
       lappend load_pins $pin1
     }
   }
-  insert_buffer_cmd $buffer_name $buffer_cell $net $load_pins $buffer_out_net_name
+  if { $buffer_cell != "NULL" \
+	 &&  $net  != "NULL" } {
+    insert_buffer_cmd $buffer_name $buffer_cell $net $load_pins $buffer_out_net_name
+  }
 }
 
 # sta namespace end.