Export button grayed out…

[Hitecrd]

Sometimes when I put a clip on the time line (from the bin) and do not set any "in" or "out" points, the export "button" remains grayed out. Once I set an "in" point it becomes active. This has happened a few times, but I am unable to reproduce it and make it happen. I typically always export the entire timeline and have never had this problem with any version prior to
4.51c.

Thanks.

P5B Deluxe/WiFi-AP
2Gb RAM