Object reference not set to an instance of an object error

I have the following code which is throwing an "object reference not set to an instance of an object error".  How can I change it to not give this exception?  The code is supposed to check for SecondaryWageAmount and if it's there, add it to PrimaryWageAmount otherwise just return PrimaryWageAmount

            decimal returnValue;
            returnValue = HasSecondary ?
                               PrimaryWageAmount + SecondaryWageAmount
                            : PrimaryWageAmount;
            return returnValue;

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