1 files changed, 3 insertions, 0 deletions
diff --git a/io.c b/io.c
index 637dfeae3..ef4649992 100644
--- a/io.c
+++ b/io.c
@@ -2133,6 +2133,9 @@ rb_io_binmode(io)
     OpenFile *fptr;
 
     GetOpenFile(io, fptr);
+    if ((fptr->mode & FMODE_BINMODE) && READ_DATA_BUFFERED(fptr->f)) {
+	rb_raise(rb_eIOError, "buffer already filled with text-mode content");
+    }
 #ifdef __human68k__
     if (fptr->f)
 	fmode(fptr->f, _IOBIN);