Need help urgently ICT ! ?

When i was producing the staff table in microsoft access database in the field that is asking about i set the validation rule to " M" or " F" when i was typing the details i dont know why when i typed F to one of the staff the window pop up to the screen saying that the details that i am typing are worng and please type " M" or " F" into the cells or something like that i cant remember the exact words. but anyway when i am typing " M" it accepts but when " F" it refuses to do it. Does anyone have any idea what i did wrong and how can i change it to work.? Thankis :)