diff --git a/src/fns.c b/src/fns.c
index 913705fcf80..dd05f1f4ca7 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -603,6 +603,8 @@ concat (nargs, args, target_type, last_special)
 	}
 
       result_len += len;
+      if (result_len < 0)
+	error ("String overflow");
     }
 
   if (! some_multibyte)