diff --git a/elab_expr.cc b/elab_expr.cc
index a7327475d..91d2a9a2a 100644
--- a/elab_expr.cc
+++ b/elab_expr.cc
@@ -655,6 +655,14 @@ NetExpr* PEConcat::elaborate_expr(Design*des, NetScope*scope,
 		  des->errors += 1;
 	    }
 
+	    if (rep->value().is_negative()) {
+		  cerr << get_fileline() << ": error: Concatenation repeat "
+		       << "may not be negative (" << rep->value().as_long()
+		       << ")." << endl;
+		  des->errors += 1;
+		  return 0;
+	    }
+
 	    repeat = rep;
       }
 
diff --git a/elab_net.cc b/elab_net.cc
index 65365bf3d..ab3b40334 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_negative()) {
+		  cerr << get_fileline() << ": error: Concatenation repeat "
+		       << "may not be negative (" << erep->value().as_long()
+		       << ")." << endl;
+		  des->errors += 1;
+		  return 0;
+	    }
+
 	    repeat = erep->value().as_ulong();
 	    delete etmp;
 
diff --git a/verinum.cc b/verinum.cc
index d7ed2a25a..0f9907389 100644
--- a/verinum.cc
+++ b/verinum.cc
@@ -429,6 +429,11 @@ bool verinum::is_zero() const
       return true;
 }
 
+bool verinum::is_negative() const
+{
+      return (bits_[nbits_-1] == V1) && has_sign();
+}
+
 verinum pad_to_width(const verinum&that, unsigned width)
 {
       if (that.len() >= width)
diff --git a/verinum.h b/verinum.h
index f400a1a5c..29477f7d2 100644
--- a/verinum.h
+++ b/verinum.h
@@ -71,6 +71,7 @@ class verinum {
 	// A number is "defined" if there are no x or z bits in its value.
       bool is_defined() const;
       bool is_zero() const;
+      bool is_negative() const;
 
 	// A number is "a string" if its value came directly from
 	// an ASCII description instead of a number value.