diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,8 @@
+### 0.26.15
+
++ Fix a code generation mistake when tranferring GClosure arguments of
+  unknown type.
+
 ### 0.26.12
 
 + Add support for the .gir format dialect generated by vala.
diff --git a/haskell-gi.cabal b/haskell-gi.cabal
--- a/haskell-gi.cabal
+++ b/haskell-gi.cabal
@@ -1,5 +1,5 @@
 name:                haskell-gi
-version:             0.26.14
+version:             0.26.15
 synopsis:            Generate Haskell bindings for GObject Introspection capable libraries
 description:         Generate Haskell bindings for GObject Introspection capable libraries. This includes most notably
                      GTK, but many other libraries in the GObject ecosystem provide introspection data too.
diff --git a/lib/Data/GI/CodeGen/Conversions.hsc b/lib/Data/GI/CodeGen/Conversions.hsc
--- a/lib/Data/GI/CodeGen/Conversions.hsc
+++ b/lib/Data/GI/CodeGen/Conversions.hsc
@@ -219,11 +219,11 @@
 hClosureToF :: Transfer -> Maybe Type -> CodeGen e Constructor
 -- Untyped closures
 hClosureToF transfer Nothing =
-  if transfer == TransferEverything
-  then return $ M "B.GClosure.disownGClosure"
-  -- We cast the point here because the foreign type for untyped
+  -- We cast the pointer here because the foreign type for untyped
   -- closures is always represented as Ptr (GClosure ()), while the
   -- corresponding Haskell type is the parametric "GClosure a".
+  if transfer == TransferEverything
+  then return $ M "FP.castPtr <$> B.GClosure.disownGClosure"
   else return $ M "unsafeManagedPtrCastPtr"
 -- Typed closures
 hClosureToF transfer (Just _) =
@@ -931,12 +931,14 @@
 -- to the foreign type, but in some cases, such as callbacks with
 -- closure arguments, this does not hold, as we omit the closure
 -- arguments. This function returns a type which is actually
--- isomorphic. There is another case this function deals with: for
--- convenience untyped `TGClosure` types have a type variable on the
--- Haskell side when they are arguments to functions, but we do not
--- want this when they appear as arguments to callbacks/signals, or
--- return types of properties, as it would force the type synonym/type
--- family to depend on the type variable.
+-- isomorphic.
+--
+-- There is another case this function deals with: for convenience
+-- untyped `TGClosure` types have a type variable on the Haskell side
+-- when they are arguments to functions, but we do not want this when
+-- they appear as arguments to callbacks/signals, or return types of
+-- properties, as it would force the type synonym/type family to
+-- depend on the type variable.
 isoHaskellType :: Type -> CodeGen e TypeRep
 isoHaskellType (TGClosure Nothing) =
   return $ "GClosure" `con` [con0 "()"]
diff --git a/lib/Data/GI/GIR/Type.hs b/lib/Data/GI/GIR/Type.hs
--- a/lib/Data/GI/GIR/Type.hs
+++ b/lib/Data/GI/GIR/Type.hs
@@ -115,6 +115,8 @@
 parseFundamentalType "GLib" "Variant" = return TVariant
 parseFundamentalType "GObject" "ParamSpec" = return TParamSpec
 parseFundamentalType "GObject" "Value" = return TGValue
+-- Used by vala, see https://github.com/haskell-gi/haskell-gi/issues/453
+parseFundamentalType "GLib" "ObjectPath" = return (TBasicType TUTF8)
 parseFundamentalType "GObject" "Closure" = parseClosure
 -- A TInterface type (basically, everything that is not of a known type).
 parseFundamentalType ns n = resolveQualifiedTypeName (Name ns n)