diff --git a/elaborate.cc b/elaborate.cc
index fc834585e..04a0fb397 100644
--- a/elaborate.cc
+++ b/elaborate.cc
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: elaborate.cc,v 1.112 1999/10/08 17:27:23 steve Exp $"
+#ident "$Id: elaborate.cc,v 1.113 1999/10/08 17:48:08 steve Exp $"
 #endif
 
 /*
@@ -945,8 +945,25 @@ NetNet* PEIdent::elaborate_net(Design*des, const string&path,
 
       if (msb_ && lsb_) {
 	    verinum*mval = msb_->eval_const(des, path);
-	    assert(mval);
+	    if (mval == 0) {
+		  cerr << msb_->get_line() << ": error: unable to "
+			"evaluate constant expression: " << *msb_ <<
+			endl;
+		  des->errors += 1;
+		  return 0;
+	    }
+
 	    verinum*lval = lsb_->eval_const(des, path);
+	    if (lval == 0) {
+		  cerr << lsb_->get_line() << ": error: unable to "
+			"evaluate constant expression: " << *lsb_ <<
+			endl;
+		  delete mval;
+		  des->errors += 1;
+		  return 0;
+	    }
+
+	    assert(mval);
 	    assert(lval);
 	    unsigned midx = sig->sb_to_idx(mval->as_long());
 	    unsigned lidx = sig->sb_to_idx(lval->as_long());
@@ -2570,6 +2587,9 @@ Design* elaborate(const map<string,Module*>&modules,
 
 /*
  * $Log: elaborate.cc,v $
+ * Revision 1.113  1999/10/08 17:48:08  steve
+ *  Support + in constant expressions.
+ *
  * Revision 1.112  1999/10/08 17:27:23  steve
  *  Accept adder parameters with different widths,
  *  and simplify continuous assign construction.
diff --git a/eval.cc b/eval.cc
index a98308c15..3fcba5fd2 100644
--- a/eval.cc
+++ b/eval.cc
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: eval.cc,v 1.8 1999/09/20 02:21:10 steve Exp $"
+#ident "$Id: eval.cc,v 1.9 1999/10/08 17:48:09 steve Exp $"
 #endif
 
 # include  "PExpr.h"
@@ -41,6 +41,12 @@ verinum* PEBinary::eval_const(const Design*des, const string&path) const
       verinum*res;
 
       switch (op_) {
+	  case '+': {
+		long lv = l->as_long();
+		long rv = r->as_long();
+		res = new verinum(lv+rv, l->len());
+		break;
+	  }
 	  case '-': {
 		long lv = l->as_long();
 		long rv = r->as_long();
@@ -92,6 +98,9 @@ verinum* PETernary::eval_const(const Design*, const string&) const
 
 /*
  * $Log: eval.cc,v $
+ * Revision 1.9  1999/10/08 17:48:09  steve
+ *  Support + in constant expressions.
+ *
  * Revision 1.8  1999/09/20 02:21:10  steve
  *  Elaborate parameters in phases.
  *