diff --git a/gcc/gensupport.cc b/gcc/gensupport.cc
index 959d1d9c83cf397fcb344e8d3db0f339a967587f..5c5f1cf4781551d3db95103c19cd1b70d98f4f73 100644
--- a/gcc/gensupport.cc
+++ b/gcc/gensupport.cc
@@ -619,6 +619,9 @@ public:
      [ns..ns + len) should equal XSTR (rtx, 0).  */
   conlist (const char *ns, unsigned int len, bool numeric)
   {
+    if (len == 0)
+      return;
+
     /* Trim leading whitespaces.  */
     while (ISBLANK (*ns))
       {