1 files changed, 6 insertions, 3 deletions

diff --git a/src/keymap.c b/src/keymap.c
index 6930ed08d8c..f5aea3b3902 100644
--- a/src/keymap.c
+++ b/src/keymap.c
@@ -841,7 +841,7 @@ static Lisp_Object
 store_in_keymap (keymap, idx, def)
      Lisp_Object keymap;
      register Lisp_Object idx;
-     register Lisp_Object def;
+     Lisp_Object def;
 {
   /* Flush any reverse-map cache.  */
   where_is_cache = Qnil;
@@ -1226,8 +1226,11 @@ binding KEY to DEF is added at the front of KEYMAP.  */)
       if (!CONSP (keymap))
         /* We must use Fkey_description rather than just passing key to
            error; key might be a vector, not a string.  */
-        error ("Key sequence %s uses invalid prefix characters",
-               SDATA (Fkey_description (key, Qnil)));
+        error ("Key sequence %s starts with non-prefix key %s",
+               SDATA (Fkey_description (key, Qnil)),
+               SDATA (Fkey_description (Fsubstring (key, make_number (0),
+                                                    make_number (idx)),
+                                        Qnil)));
     }
 }