1 files changed, 2 insertions, 2 deletions

diff --git a/src/fns.c b/src/fns.c
index 404f9976544..51eec4d472c 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -731,8 +731,8 @@ concat (nargs, args, target_type, last_special)
                 CHECK_NUMBER (elt, 0);
                 if (SINGLE_BYTE_CHAR_P (XINT (elt)))
                   {
-                    XSTRING (val)->data[toindex++] = XINT (elt);
-                    toindex_byte++;
+                    XSTRING (val)->data[toindex_byte++] = XINT (elt);
+                    toindex++;
                   }
                 else
                   /* If we have any multibyte characters,