diff --git a/pform.cc b/pform.cc
index 56dcd0349..ecb81b721 100644
--- a/pform.cc
+++ b/pform.cc
@@ -567,6 +567,9 @@ PWire*pform_get_make_wire_in_scope(perm_string name, NetNet::Type net_type, NetN
 	    cur = new PWire(name, net_type, port_type, vt_type);
 	    pform_put_wire_in_scope(name, cur);
       } else {
+	      // If this is a duplicate wire, the data type has already
+	      // been set, then return NULL.
+	    if (cur->get_data_type() != IVL_VT_NO_TYPE) return 0;
 	    bool rc = cur->set_wire_type(net_type);
 	    assert(rc);
 	    rc = cur->set_data_type(vt_type);
@@ -3144,6 +3147,7 @@ template <class T> static void pform_set2_data_type(const struct vlltype&li, T*d
       }
 
       PWire*net = pform_get_make_wire_in_scope(name, net_type, NetNet::NOT_A_PORT, base_type);
+      assert(net);
       net->set_data_type(data_type);
       pform_bind_attributes(net->attributes, attr, true);
 }
@@ -3160,9 +3164,14 @@ static void pform_set_enum(const struct vlltype&li, enum_type_t*enum_type,
 			   perm_string name, NetNet::Type net_type,
 			   std::list<named_pexpr_t>*attr)
 {
-      (void) li; // The line information is not currently needed.
       PWire*cur = pform_get_make_wire_in_scope(name, net_type, NetNet::NOT_A_PORT, enum_type->base_type);
-      assert(cur);
+	// A NULL is returned for a duplicate enumeration.
+      if (! cur) {
+	    cerr << li.get_fileline() << ": error: Found duplicate "
+		 << "enumeration named " << name << "." << endl;
+	    error_count += 1;
+	    return;
+      }
 
       cur->set_signed(enum_type->signed_flag);
 
diff --git a/pform_class_type.cc b/pform_class_type.cc
index 44f0d3555..ec2f99de4 100644
--- a/pform_class_type.cc
+++ b/pform_class_type.cc
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012 Picture Elements, Inc.
+ * Copyright (c) 2012-2014 Picture Elements, Inc.
  *    Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
@@ -25,6 +25,7 @@
 static void pform_set_class_type(class_type_t*class_type, perm_string name, NetNet::Type net_type, list<named_pexpr_t>*attr)
 {
       PWire*net = pform_get_make_wire_in_scope(name, net_type, NetNet::NOT_A_PORT, IVL_VT_CLASS);
+      assert(net);
       net->set_data_type(class_type);
       pform_bind_attributes(net->attributes, attr, true);
 }
diff --git a/pform_string_type.cc b/pform_string_type.cc
index 4d1f1d2dc..50a7f4fce 100644
--- a/pform_string_type.cc
+++ b/pform_string_type.cc
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012-2013 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2012-2014 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -24,6 +24,7 @@
 static void pform_set_string_type(const string_type_t*, perm_string name, NetNet::Type net_type, list<named_pexpr_t>*attr)
 {
       PWire*net = pform_get_make_wire_in_scope(name, net_type, NetNet::NOT_A_PORT, IVL_VT_STRING);
+      assert(net);
       pform_bind_attributes(net->attributes, attr, true);
 }
 
diff --git a/pform_struct_type.cc b/pform_struct_type.cc
index 4a4d91251..766fc77e3 100644
--- a/pform_struct_type.cc
+++ b/pform_struct_type.cc
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011-2012 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2011-2014 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -67,6 +67,7 @@ static void pform_set_packed_struct(struct_type_t*struct_type, perm_string name,
       ivl_variable_type_t base_type = struct_type->figure_packed_base_type();
 
       PWire*net = pform_get_make_wire_in_scope(name, net_type, NetNet::NOT_A_PORT, base_type);
+      assert(net);
       net->set_data_type(struct_type);
       pform_bind_attributes(net->attributes, attr, true);
 }
@@ -99,6 +100,7 @@ static void pform_makewire(const struct vlltype&li,
       ivl_variable_type_t base_type = struct_type->figure_packed_base_type();
 
       PWire*cur = pform_get_make_wire_in_scope(name, NetNet::WIRE, ptype, base_type);
+      assert(cur);
       FILE_NAME(cur, li);
       cur->set_data_type(struct_type);
 }