CST MWS frequency must be greater than zero error

I am using CST MWS 2009, I am trying to run a parameter sweep with a frequency range of 4-10GHz. At the end of each sweep an error appears saying the frequency must be greater than zero, if I click ok the error goes and the sweep continues, until it reaches the end of of that sweep and needs to increment the value of the parameter again.
It means that if I want to run a sweep I have to be there to click okay every time it wants to increment the parameter.
Any ideas what could be causing this error despite the frequency range being set?

Thanks

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