diff --git a/Source/cmVTKWrapJavaCommand.cxx b/Source/cmVTKWrapJavaCommand.cxx
index eba472388..2a91878a2 100644
--- a/Source/cmVTKWrapJavaCommand.cxx
+++ b/Source/cmVTKWrapJavaCommand.cxx
@@ -69,6 +69,11 @@ bool cmVTKWrapJavaCommand::InitialPass(std::vector<std::string>& args)
       j != args.end(); ++j)
     {   
     cmMakefile::SourceMap::iterator l = Classes.find(*j);
+    if (l == Classes.end())
+      {
+      this->SetError("bad source list passed to VTKWrapJavaCommand");
+      return false;
+      }
     for(std::vector<cmSourceFile>::iterator i = l->second.begin(); 
         i != l->second.end(); i++)
       {
diff --git a/Source/cmVTKWrapPythonCommand.cxx b/Source/cmVTKWrapPythonCommand.cxx
index 0c1d6a2b7..705a8a451 100644
--- a/Source/cmVTKWrapPythonCommand.cxx
+++ b/Source/cmVTKWrapPythonCommand.cxx
@@ -70,6 +70,11 @@ bool cmVTKWrapPythonCommand::InitialPass(std::vector<std::string>& args)
       j != args.end(); ++j)
     {   
     cmMakefile::SourceMap::iterator l = Classes.find(*j);
+    if (l == Classes.end())
+      {
+      this->SetError("bad source list passed to VTKWrapPythonCommand");
+      return false;
+      }
     for(std::vector<cmSourceFile>::iterator i = l->second.begin(); 
         i != l->second.end(); i++)
       {
diff --git a/Source/cmVTKWrapTclCommand.cxx b/Source/cmVTKWrapTclCommand.cxx
index fcf5c452c..8c76017a3 100644
--- a/Source/cmVTKWrapTclCommand.cxx
+++ b/Source/cmVTKWrapTclCommand.cxx
@@ -101,6 +101,11 @@ bool cmVTKWrapTclCommand::InitialPass(std::vector<std::string>& args)
         j != sources.end(); ++j)
       {   
       cmMakefile::SourceMap::iterator l = Classes.find(*j);
+      if (l == Classes.end())
+	{
+	this->SetError("bad source list passed to VTKWrapTclCommand");
+	return false;
+	}
       for(std::vector<cmSourceFile>::iterator i = l->second.begin(); 
           i != l->second.end(); i++)
         {