Expression that yield variant data type cannot be sued to define calculated Columns.

Hello Everyone,

I am getting below error,

Error Descriptions:Expression that yield variant data type cannot be sued to define calculated Columns.

Data type:

Start Year=Whole Number

Start Month= Whole Number

Current Year= Whole Number

Current Month= Whole Number

Valid Price1 = IF((' Contracts'[Start Year]+[ Start Month]/12)<=([AUP Current Year]+[AUP Current Month]/12),[Monthly Discounted Price],"Contract Not Yet Started")

Does any has any suggestion?

Thanks