1 files changed, 4 insertions, 0 deletions

diff --git a/io.c b/io.c
index 653034dd58..9900118818 100644
--- a/io.c
+++ b/io.c
@@ -2398,23 +2398,27 @@ read_all(rb_io_t *fptr, long siz, VALUE str)
     int cr;
 
     if (NEED_READCONV(fptr)) {
+	int first = !NIL_P(str);
 	SET_BINARY_MODE(fptr);
 	io_setstrbuf(&str,0);
         make_readconv(fptr, 0);
         while (1) {
             VALUE v;
             if (fptr->cbuf.len) {
+		if (first) rb_str_set_len(str, first = 0);
                 io_shift_cbuf(fptr, fptr->cbuf.len, &str);
             }
             v = fill_cbuf(fptr, 0);
             if (v != MORE_CHAR_SUSPENDED && v != MORE_CHAR_FINISHED) {
                 if (fptr->cbuf.len) {
+		    if (first) rb_str_set_len(str, first = 0);
                     io_shift_cbuf(fptr, fptr->cbuf.len, &str);
                 }
                 rb_exc_raise(v);
             }
             if (v == MORE_CHAR_FINISHED) {
                 clear_readconv(fptr);
+		if (first) rb_str_set_len(str, first = 0);
                 return io_enc_str(str, fptr);
             }
         }