Using the when event in a command button

I have a command button on a form that should not be accessible unless there is a table selected. I am attempting to use a expression that will evaluate to false in the WHEN event when no table is selected:

NOT EMPTY(thisform.combo1.Value)

The message I get is "Error in line 7 of Form1.Command1.When (record 7): Unrecognized command verb."

Even with a .f. as the expression I get the same message. Am I giving the fox something she doesn't like or am I working with some bad assumption?

Thanks in advance for any help,

Joe

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