Unable to generate a temporary class (result=1).

I couldn't find which thread to relate this problem. So I am posting it here.
Objective:
I have created a webservice, Trip.asmx, that exposes a web method whose signature is public FList[] Find(FCriteria criteria)
The object FList is derived from ICollection. (And yes FList is marked serializable)


Problem:
When I make a SOAP request to this API, the following exception is thrown

System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.InvalidOperationException: Unable to generate a temporary class (result=1). error CS1026: ) expected

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