diff --git a/elab_expr.cc b/elab_expr.cc
index 90c1937fc..d452b31e4 100644
--- a/elab_expr.cc
+++ b/elab_expr.cc
@@ -716,6 +716,15 @@ NetExpr* PEConcat::elaborate_expr(Design*des, NetScope*scope,
 		  des->errors += 1;
 	    }
 
+	    if (!rep->value().is_defined()) {
+		  cerr << get_fileline() << ": error: Concatenation repeat "
+		       << "may not be undefined (" << rep->value()
+		       << ")." << endl;
+		  des->errors += 1;
+		  concat_depth -= 1;
+		  return 0;
+	    }
+
 	    if (rep->value().is_negative()) {
 		  cerr << get_fileline() << ": error: Concatenation repeat "
 		       << "may not be negative (" << rep->value().as_long()
diff --git a/elab_net.cc b/elab_net.cc
index 97fcb76cd..2e9224483 100644
--- a/elab_net.cc
+++ b/elab_net.cc
@@ -1578,6 +1578,14 @@ NetNet* PEConcat::elaborate_net(Design*des, NetScope*scope,
 		  return 0;
 	    }
 
+	    if (!erep->value().is_defined()) {
+		  cerr << get_fileline() << ": error: Concatenation repeat "
+		       << "may not be undefined (" << erep->value()
+		       << ")." << endl;
+		  des->errors += 1;
+		  return 0;
+	    }
+
 	    if (erep->value().is_negative()) {
 		  cerr << get_fileline() << ": error: Concatenation repeat "
 		       << "may not be negative (" << erep->value().as_long()