RangeValidator1 Error

I'm using  RangeValidator1 control to validate that user entered integer value, but I got his error
"The value '' of the Maximum Value property of 'RangeValidator1' cannot be converted to type 'Integer'"
I tried to convert it using this code:
        TextBox grabber_statusTextBox = (TextBox)FormView1.FindControl("grabber_statusTextBox");
        RangeValidator RangeValidator1 = (RangeValidator)FormView1.FindControl("RangeValidator1");
        RangeValidator1.MaximumValue = grabber_statusTextBox.ToString();
I put it in the form_load but it did not work

 
 
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