Strange error when using direct DPair instead of (... ** ...) syntax of a derived generator return type

[buzden]

I had a definitions and a derived generator

data NameIsNew : (funs : Funs) -> (vars : Vars) -> UniqNames funs vars -> String -> Type

genNewName : Fuel -> (Fuel -> Gen MaybeEmpty String) =>
             (funs : Funs) -> (vars : Vars) -> (names : UniqNames funs vars) ->
             Gen MaybeEmpty (s ** NameIsNew funs vars names s)

and all worked, but once I change the return type to be in form of

             Gen MaybeEmpty $ DPair String $ NameIsNew funs vars names

derivation suddenly crashes with

Error: While processing right hand side of genNewName. Error during reflection: Target type's argument must be a variable name

with non-var expression being String.

Suddenly, replacing the return type with

             Gen MaybeEmpty $ DPair String $ \x => NameIsNew funs vars names x

helps with this.