1 files changed, 2 insertions, 2 deletions

diff --git a/src/fns.c b/src/fns.c
index 217e97ffd56..9874b4dfa48 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -714,7 +714,7 @@ concat (nargs, args, target_type, last_special)
             }
           toindex_byte += thislen_byte;
           toindex += thisleni - combined;
-          SCHARS (val) -= combined;
+          STRING_SET_CHARS (val, SCHARS (val) - combined);
         }
       /* Copy a single-byte string to a multibyte string.  */
       else if (STRINGP (this) && STRINGP (val))
@@ -804,7 +804,7 @@ concat (nargs, args, target_type, last_special)
                         && toindex_byte > 0
                         && count_combining (SDATA (val),
                                             toindex_byte, toindex_byte - 1))
-                      SCHARS (val)--;
+                      STRING_SET_CHARS (val, SCHARS (val) - 1);
                     else
                       toindex++;
                   }