Strange compiler error: Cannot unify constraint with constraint1 - idris

Question

I am trying to write a library in Idris to work with categories and functors. For my use case, each type can be a category in at most one way (and I would like to use overloading for id and .), so using an interface suits my needs:
infixr 9 .
interface CategoryI (o : Type) where
data Hom : o -> o -> Type
(.) : {a : o} -> {b : o} -> {c : o} -> Hom b c -> Hom a b -> Hom a c
id : (a : o) -> Hom a a
However, for functors, I need a datatype rather than an interface, since there can be multiple functors between the same two categories that I would like to consider.
data Functor : Type -> Type -> Type where
MkFunctor : (CategoryI s, CategoryI t) =>
(f : s -> t)
-> (Hom x y -> Hom (f x) (f y))
-> Functor s t
To make use of this, I have written accesser functions:
src : Functor s t -> Type
src (MkFunctor fo fm) = s
tgt : Functor s t -> Type
tgt (MkFunctor fo fm) = t
f_obj : (CategoryI s, CategoryI t) => Functor s t -> (s -> t)
f_obj (MkFunctor fo fm) = fo
But I am having problems with the following:
f_map : (CategoryI s, CategoryI t) => (f : Functor s t)
-> (Hom x y -> Hom ((f_obj f) x) ((f_obj f) y))
f_map (MkFunctor fo fm) = fm
The compiler complains:
When checking right hand side of f_map with expected type
Hom x y -> Hom (f_obj (MkFunctor fo fm) x) (f_obj (MkFunctor fo fm) y)
Type mismatch between
Hom x y -> Hom (fo x) (fo y) (Type of fm x y)
and
Hom x y -> Hom (fo x) (fo y) (Expected type)
Specifically:
Type mismatch between
constraint1
and
constraint
I can't even determine what the compiler is complaining abut here. IT somehow can't unify two constraints (which ones?), but the constraints on my constructor and on my f_map function are the same, so I don't see what the issue is.
How can I resolve this issue?