1 files changed, 3 insertions, 1 deletions
diff --git a/string.c b/string.c
index eebc61cd10..d0ccce7671 100644
--- a/string.c
+++ b/string.c
@@ -1314,7 +1314,9 @@ rb_str_update(str, beg, len, val)
 	memmove(RSTRING(str)->ptr+beg, RSTRING(val)->ptr, RSTRING(val)->len);
     }
     RSTRING(str)->len += RSTRING(val)->len - len;
-    RSTRING(str)->ptr[RSTRING(str)->len] = '\0';
+    if (RSTRING(str)->ptr) {
+	RSTRING(str)->ptr[RSTRING(str)->len] = '\0';
+    }
     OBJ_INFECT(str, val);
 }