Type family equation violates injectivity?

When I use v8.6.3 of GHC under Ubuntu to install the inline-r package I get the error "Type family equation violates injectivity annotation," and a type variable on the LHS cannot be inferred from the RHS, due to the lack of injectivity (I suppose). On the other hand, v8.0.2 of GHC (shipped with Haskell Platform under Ubuntu) does not have this problem (it has other problems). Has something changed in the latest version of the compiler that might cause this? Possible work-around? FYI, the line that triggers the error is: type instance G.Mutable (W t ty s) = Mutable.W t ty The variable that cannot be inferred is 's'. Thanks, Dominick