diff --git a/elab_expr.cc b/elab_expr.cc
index 336cdb60b..037b04da2 100644
--- a/elab_expr.cc
+++ b/elab_expr.cc
@@ -428,6 +428,13 @@ NetExpr* PEConcat::elaborate_expr(Design*des, NetScope*scope, bool) const
 		  des->errors += 1;
 	    }
 
+	    if (!rep->value().is_defined()) {
+		  cerr << get_line() << ": error: Concatenation repeat "
+		       << "may not be undefined (" << rep->value()
+		       << ")." << endl;
+		  des->errors += 1;
+	    }
+
 	    repeat = rep;
       }
 
diff --git a/elab_net.cc b/elab_net.cc
index 6e443b923..098e3f7a7 100644
--- a/elab_net.cc
+++ b/elab_net.cc
@@ -1341,6 +1341,14 @@ NetNet* PEConcat::elaborate_net(Design*des, NetScope*scope,
 		  des->errors += 1;
 		  return 0;
 	    }
+
+	    if (!erep->value().is_defined()) {
+		  cerr << get_line() << ": error: Concatenation repeat "
+		       << "may not be undefined (" << erep->value()
+		       << ")." << endl;
+		  des->errors += 1;
+		  return 0;
+	    }
       }
 
 	/* The operands of the concatenation must contain all