1 files changed, 5 insertions, 2 deletions

diff --git a/src/keymap.c b/src/keymap.c
index 7ff3f471b0f..c609ed6c4e4 100644
--- a/src/keymap.c
+++ b/src/keymap.c
@@ -1186,8 +1186,11 @@ 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)));
     }
 }