1 files changed, 4 insertions, 1 deletions
diff --git a/io.c b/io.c
index 074d2c50f7..f404b539a4 100644
--- a/io.c
+++ b/io.c
@@ -2084,7 +2084,10 @@ io_setstrbuf(VALUE *str, long len)
 	VALUE s = StringValue(*str);
 	long clen = RSTRING_LEN(s);
 	if (clen >= len) {
-	    if (clen != len) rb_str_set_len(s, len);
+	    if (clen != len) {
+		rb_str_modify(s);
+		rb_str_set_len(s, len);
+	    }
 	    return;
 	}
 	len -= clen;