WebMatrix :: Can Convert String To Decimal/ 'string' Does Not Contain A Definition For 'AsDecimal'

I'm trying to convert a string to decimal but I get this exception:

Exception Details:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: 'string' does not contain a definition for 'AsDecimal'

Here is the code:

decimal PriceHT = 0;

var PriceRequest= "SELECT * FROM Price_" + HttpContext.Current.Session["PriceTableId"] ;
foreach (var Price in Database.OpenFile("Base.sdf").Query(PriceRequest)){
PriceHT = PrixceHT + (Price.PT).AsDecimal(); //Price.PT returns a string
}