diff --git a/io.c b/io.c
index e5fac3c18c..b615aca8c1 100644
--- a/io.c
+++ b/io.c
@@ -1306,7 +1306,7 @@ rb_io_from_fd(int f)
     rb_io_t *fptr;
     RB_IO_POINTER(io, fptr);
 
-    fptr->mode &= ~FMODE_PREP;
+    fptr->mode |= FMODE_PREP;
 
     return io;
 }