How can i multiply a decimal? with a double?

Question

 

the properties :

public decimal? Percentage { get; set; } 

public double Amount { get; set;

 

foreach (var item in updatethis)

{

    item.Amount = (CPP.Percentage * CPP.Amount);

}

 Im getting this error Operator '*' cannot be applied to operands of type 'decimal?' and 'double'

 

how can i solve this ? thank you in advance :)