Data Validation

I have been working with data validation for some time. I have someone who is creating a list and they want two options; x or just a blank cell. On my computer, it works fine. In data validation, I choose list and then type in x,(space). This allows me to choose an x or a space from the drop down. On her computer, doing the exact same thing, she only gets the option to choose the x, and not the space. I assume this has to do with settings or options on her Excel. I have spent some time trying to figure this out and I am not having any luck. Any suggestions?