(concat): Check STRINGP before increasing result_len_byte.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/fns.c b/src/fns.c
index c1e29b82e96..69705af4319 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -456,7 +456,7 @@ concat (nargs, args, target_type, last_special)
                 if (this_len_byte > 1)
                   some_multibyte = 1;
               }
-          else
+          else if (STRINGP (this))
             {
               result_len_byte += XSTRING (this)->size_byte;
               if (STRING_MULTIBYTE (this))