DC Analysis

I am trying to run a Probe DC Analysis on a circuit, but whenever I try to start the run, I receive a Source Not Found error. How do I get past this?

The problem that is occurring is that there in no input defined for the Variable 1 field. When this error occurs, the DC analysis limits should appear behind the error. This dialog box can also be accessed by hitting F9. The default for Variable 1 is blank, which is causing the error. You must fill in a variable for this field that is to be swept in DC analysis. Variable 2 can be left empty if only one source is to be swept. Fill in a variable, hit F2, and it should run fine.