[AccessD] Access #Num! When Calculating A Field

To all,
 
I have two bound fields on a form (txtNoOfCases & txtNoOfErrors), I then have an unbound textbox set to Percent with the control source being 
 
=[txtNoOfErrors]/[txtNoOfCases]
 
But when you first open the form the unbound textbox has #Num! in it until you enter a value in either of the two fields.  How can I make the unbound textbox say 0.00 instead of #Num! until data is entered ?
 
Thanks in advance for you help.
 
Paul Hartland
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