1 files changed, 4 insertions, 1 deletions
diff --git a/io.c b/io.c
index 79c3024538..474562a944 100644
--- a/io.c
+++ b/io.c
@@ -2157,7 +2157,10 @@ io_read(int argc, VALUE *argv, VALUE io)
     rb_scan_args(argc, argv, "02", &length, &str);
 
     if (NIL_P(length)) {
-	if (!NIL_P(str)) StringValue(str);
+	if (!NIL_P(str)){
+	    StringValue(str);
+	    rb_str_modify(str);
+	}
 	GetOpenFile(io, fptr);
 	rb_io_check_char_readable(fptr);
 	return read_all(fptr, remain_size(fptr), str);