Server Error in '/WebServicesTesting' Application. Unable to cast object of type 'System.Web.UI.WebControls.TextBox' to type 'System.IConvertible'

I'm trying to convert the value entered to decimal.  So for example I enter 38.99, I want to conver it to decimal format in code behind but obviously I'm not doing this right:
private decimal cardAmount;
cardAmount = Convert.ToDecimal(txtCreditCardAmount);
I get that error on cast.

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