Integration Services :: CLR Types For Return Value Do Not Match

I am trying to create a CLR function to call a webservice, the CLR function return data type is double, whether I try to create this as a table valued funcion or a scalar to return a distance travelled value I am receiving the error below.

I've tried changing data types around in the CLR side and the SQL side but keep receiving the same error message:

[Microsoft.SqlServer.Server.SqlFunction(Name = "DistanceCalc")]
public static Double DistanceCalc(Double SrcLat, Double SrcLong,
Double DestLat, Double DestLong)
{
MileageWS ws = new MileageWS();

[Code] ....

Error received when try to Create function ...
1, Level 16, State 2, Procedure pcMiler, Line 6
CREATE FUNCTION for "pcMiler" failed because T-SQL and CLR types for return value do not match.