(concat): Check STRINGP before increasing result_len_byte.

diff --git a/src/fns.c b/src/fns.c
index c1e29b82e96144d309a7407583ea3cf402e60203..69705af4319bde5d19810b429fd4bc973f33dc40 100644 (file)
--- 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))