diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+### 0.22.6
+
++ Fix generated IsX typeclasses for non-GObject interfaces.
+
 ### 0.22.5
 
 + Add support for inheriting overloading info.
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:             0.22.5
+version:             0.22.6
 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/CodeGen.hs b/lib/Data/GI/CodeGen/CodeGen.hs
--- a/lib/Data/GI/CodeGen/CodeGen.hs
+++ b/lib/Data/GI/CodeGen/CodeGen.hs
@@ -381,8 +381,17 @@
     writeHaddock DocBeforeSymbol ("Type class for types which implement `"
                                   <> name' <> "`.")
 
-    bline $ "class ManagedPtrNewtype a => " <> cls <> " a"
-    line $ "instance " <> cls <> " " <> name'
+    -- Create the IsX constraint. We cannot simply say
+    --
+    -- > type IsX o = (ManagedPtrNewtype o, O.IsDescendantOf X o)
+    --
+    -- since we sometimes need to refer to @IsX@ itself, without
+    -- applying it. We instead use the trick of creating a class with
+    -- a universal instance.
+    let constraints = "(ManagedPtrNewtype o, O.IsDescendantOf " <> name' <> " o)"
+    bline $ "class " <> constraints <> " => " <> cls <> " o"
+    bline $ "instance " <> constraints <> " => " <> cls <> " o"
+
     genWrappedPtr n (ifAllocationInfo iface) 0
 
     when (not . null . ifProperties $ iface) $ group $ do
diff --git a/lib/Data/GI/CodeGen/EnumFlags.hs b/lib/Data/GI/CodeGen/EnumFlags.hs
--- a/lib/Data/GI/CodeGen/EnumFlags.hs
+++ b/lib/Data/GI/CodeGen/EnumFlags.hs
@@ -7,6 +7,7 @@
 import Control.Monad (when, forM_)
 import Data.Monoid ((<>))
 import Data.Text (Text)
+import qualified Data.Set as S
 
 import Foreign.C (CUInt)
 import Foreign.Storable (sizeOf)
@@ -18,6 +19,19 @@
 import Data.GI.CodeGen.SymbolNaming (upperName)
 import Data.GI.CodeGen.Util (tshow)
 
+-- | Given a list of named enum members, filter out those that have
+-- the same value as a previous entry in the list.
+dropDuplicated :: [(Text, EnumerationMember)] -> [(Text, EnumerationMember)]
+dropDuplicated namedMembers = go namedMembers enumMemberValue S.empty
+  where go :: Ord c => [(a, b)] -> (b->c) -> S.Set c -> [(a, b)]
+        go [] _ _ = []
+        go ((n, m) : rest) f seen =
+          if S.member (f m) seen
+             -- already seen, discard
+          then go rest f seen
+          else (n,m) : go rest f (S.insert (f m) seen)
+
+
 genEnumOrFlags :: HaddockSection -> Name -> Enumeration -> ExcCodeGen ()
 genEnumOrFlags docSection n@(Name ns name) e = do
   -- Conversion functions expect enums and flags to map to CUInt,
@@ -61,7 +75,7 @@
             line $ "fromEnum (Another" <> name' <> " k) = k"
     blank
     indent $ do
-            forM_ members' $ \(n, m) ->
+            forM_ (dropDuplicated members') $ \(n, m) ->
                 line $ "toEnum " <> tshow (enumMemberValue m) <> " = " <> n
             line $ "toEnum k = Another" <> name' <> " k"