diff --git a/src/eval.c b/src/eval.c
index 51ec0b8..728b7d1 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -24848,8 +24848,11 @@
 		if (zero_width == regmatch.startp[0])
 		{
 		    /* avoid getting stuck on a match with an empty string */
-		    *((char_u *)ga.ga_data + ga.ga_len) = *tail++;
-		    ++ga.ga_len;
+		    i = MB_PTR2LEN(tail);
+		    mch_memmove((char_u *)ga.ga_data + ga.ga_len, tail,
+								   (size_t)i);
+		    ga.ga_len += i;
+		    tail += i;
 		    continue;
 		}
 		zero_width = regmatch.startp[0];
diff --git a/src/testdir/test69.in b/src/testdir/test69.in
index 75317b4..2510c12 100644
--- a/src/testdir/test69.in
+++ b/src/testdir/test69.in
@@ -180,6 +180,13 @@
 byteidxcomp
 
 STARTTEST
+/^substitute
+:let y = substitute('１２３', '\zs', 'a', 'g')    | put =y
+ENDTEST
+
+substitute
+
+STARTTEST
 :g/^STARTTEST/.,/^ENDTEST/d
 :1;/^Results/,$wq! test.out
 ENDTEST
diff --git a/src/testdir/test69.ok b/src/testdir/test69.ok
index 41cd9d0..af8befb 100644
--- a/src/testdir/test69.ok
+++ b/src/testdir/test69.ok
@@ -160,3 +160,7 @@
 [0, 1, 3, 4, -1]
 [0, 1, 2, 4, 5, -1]
 
+
+substitute
+a１a２a３a
+
diff --git a/src/version.c b/src/version.c
index 5556ed7..93046dd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -735,6 +735,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    323,
+/**/
     322,
 /**/
     321,