1 files changed, 5 insertions, 0 deletions
diff --git a/src/io.c b/src/io.c
index 992801a62..6f52daf4a 100644
--- a/src/io.c
+++ b/src/io.c
@@ -630,8 +630,13 @@ mrb_io_sysread(mrb_state *mrb, mrb_value io)
   if (mrb_nil_p(buf)) {
     buf = mrb_str_new(mrb, NULL, maxlen);
   }
+
+  mrb_str_modify(mrb, RSTRING(buf));
+
   if (RSTRING_LEN(buf) != maxlen) {
     buf = mrb_str_resize(mrb, buf, maxlen);
+  } else {
+    mrb_str_modify(mrb, RSTRING(buf));
   }
 
   fptr = (struct mrb_io *)io_get_open_fptr(mrb, io);