Strange error “argument type ‘lambda expression’ is not assignable to parameter type ‘string’”

During playing with EF and LINQ I run in this UI-error:

argument type ‘lambda expression’ is not assignable to parameter type ‘string’

The Compiler themselves states:

Cannot convert lambda expression to type ‘string’ because it is not a delegate type.

After some googleling I found, that a missing using directive is the problem. Inserting System.Linq fixed it