diff --git a/src/string.c b/src/string.c
index edaf1edc..e9a0095b 100644
--- a/src/string.c
+++ b/src/string.c
@@ -74,28 +74,29 @@ pic_str_ref(pic_state *pic, pic_str *str, size_t i)
 static xrope *
 xr_put(xrope *rope, size_t i, char c)
 {
-  xrope *x, *y;
-  char buf[1];
+  xrope *x, *y, *z;
+  char buf[2];
 
   if (xr_len(rope) <= i) {
     return NULL;
   }
 
   buf[0] = c;
+  buf[1] = '\0';
 
   x = xr_sub(rope, 0, i);
   y = xr_new_copy(buf, 1);
-  rope = xr_cat(x, y);
+  z = xr_cat(x, y);
   XROPE_DECREF(x);
   XROPE_DECREF(y);
 
-  x = rope;
+  x = z;
   y = xr_sub(rope, i + 1, xr_len(rope));
-  rope = xr_cat(x, y);
+  z = xr_cat(z, y);
   XROPE_DECREF(x);
   XROPE_DECREF(y);
 
-  return rope;
+  return z;
 }
 
 void