diff --git a/ChangeLog b/ChangeLog
index 3e9b6ffa0..9e0a94b10 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2023-10-10  Oliver Kiddle  <opk@zsh.org>
 
+	* 52214: Src/subst.c: allow extra byte for nul in allocation
+
 	* unposted (cf. 52166): Functions/Misc/run-help-svk:
 	remove obsolete helper for svk
 
diff --git a/Src/subst.c b/Src/subst.c
index cdbfc138a..60d850feb 100644
--- a/Src/subst.c
+++ b/Src/subst.c
@@ -1506,7 +1506,7 @@ substevalchar(char *ptr)
     }
 #ifdef MULTIBYTE_SUPPORT
     else if (isset(MULTIBYTE) && ires > 127) {
-	ptr = zhalloc(MB_CUR_MAX);
+	ptr = zhalloc(MB_CUR_MAX+1);
 	len = ucs4tomb((unsigned int)ires & 0xffffffff, ptr);
     }
     if (len <= 0)