diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 0.5.0.1 (June 2, 2017)
+
+  - Fixed an issue where using `mkFixture` with a typeclass that used the `DefaultSignatures` extension caused an internal error ([#30](https://github.com/cjdev/test-fixture/issues/30)).
+
 # 0.5.0.0 (November 28, 2016)
 
   - **Breaking**: `mkFixture` now supports constraints in the same form as a Haskell `deriving` clause, which permits “partially-applied” constraints. A new `ts` quasiquoter is provided for the purpose of writing a comma-separated list of Haskell types; see the documentation for more details ([#25](https://github.com/cjdev/test-fixture/issues/25)).
diff --git a/src/Control/Monad/TestFixture/TH.hs b/src/Control/Monad/TestFixture/TH.hs
--- a/src/Control/Monad/TestFixture/TH.hs
+++ b/src/Control/Monad/TestFixture/TH.hs
@@ -92,4 +92,4 @@
 
 import Control.Monad.TestFixture.TH.Internal (mkFixture)
 import Control.Monad.TestFixture.TH.Internal.TypesQuasi (ts)
-import Data.Default (def)
+import Data.Default.Class (def)
diff --git a/src/Control/Monad/TestFixture/TH/Internal.hs b/src/Control/Monad/TestFixture/TH/Internal.hs
--- a/src/Control/Monad/TestFixture/TH/Internal.hs
+++ b/src/Control/Monad/TestFixture/TH/Internal.hs
@@ -1,6 +1,7 @@
 {-# OPTIONS_HADDOCK hide, not-home #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TupleSections #-}
 
@@ -15,7 +16,7 @@
 import Control.Monad (join, replicateM, when, zipWithM)
 import Control.Monad.TestFixture (TestFixture, TestFixtureT, unimplemented)
 import Data.Char (isPunctuation, isSymbol)
-import Data.Default (Default(..))
+import Data.Default.Class (Default(..))
 import Data.List (foldl', nub, partition)
 import GHC.Exts (Constraint)
 import Language.Haskell.TH
@@ -130,9 +131,7 @@
   let instanceHead = AppT classType fixtureT
 
   classInfo <- reify (unappliedTypeName classType)
-  methods <- case classInfo of
-    ClassI (ClassD _ _ _ _ methods) _ -> return methods
-    _ -> fail $ "mkInstance: expected a class type, given " ++ show classType
+  methods <- classMethods classInfo
   funDecls <- traverse mkDictInstanceFunc methods
 
   return $ mkInstanceD [AppT (ConT ''Monad) mVar] instanceHead funDecls
@@ -175,8 +174,11 @@
   Given some 'Info' about a class, get its methods as 'SigD' declarations.
 -}
 classMethods :: MonadFail m => Info -> m [Dec]
-classMethods (ClassI (ClassD _ _ _ _ methods) _) = return methods
-classMethods other = fail $ "classMethods: expected a class name, given " ++ show other
+classMethods (ClassI (ClassD _ _ _ _ methods) _) = return $ removeDefaultSigs methods
+  where removeDefaultSigs = filter $ \case
+          DefaultSigD{} -> False
+          _             -> True
+classMethods other = fail $ "classMethods: expected a class type, given " ++ show other
 
 {-|
   Helper for applying `methodToField` over multiple methods using the same name
diff --git a/test-fixture.cabal b/test-fixture.cabal
--- a/test-fixture.cabal
+++ b/test-fixture.cabal
@@ -1,7 +1,7 @@
 name:
   test-fixture
 version:
-  0.5.0.0
+  0.5.0.1
 synopsis:
   Test monadic side-effects
 description:
@@ -40,7 +40,7 @@
     Control.Monad.TestFixture.TH.Internal.TypesQuasi
   build-depends:
       base >= 4.7 && < 5
-    , data-default
+    , data-default-class
     , haskell-src-exts
     , haskell-src-meta
     , mtl
@@ -61,6 +61,7 @@
   ghc-options: -Wall
   other-modules:
     Test.Control.Monad.TestFixtureSpec
+    Test.Control.Monad.TestFixture.DefaultSignaturesSpec
     Test.Control.Monad.TestFixture.THSpec
   build-depends:
       base >= 4.7 && < 5
@@ -70,6 +71,3 @@
     , mtl
     , template-haskell >= 2.10 && < 2.12
     , transformers
-  if impl(ghc >= 8.0.0.0)
-    build-depends:
-      th-to-exp
diff --git a/test/Test/Control/Monad/TestFixture/DefaultSignaturesSpec.hs b/test/Test/Control/Monad/TestFixture/DefaultSignaturesSpec.hs
new file mode 100644
--- /dev/null
+++ b/test/Test/Control/Monad/TestFixture/DefaultSignaturesSpec.hs
@@ -0,0 +1,37 @@
+{-# LANGUAGE CPP #-}
+
+#if MIN_VERSION_GLASGOW_HASKELL(8,0,0,0)
+{-# OPTIONS_GHC -fno-warn-unused-top-binds -fno-warn-redundant-constraints #-}
+#else
+{-# OPTIONS_GHC -fno-warn-unused-binds #-}
+#endif
+
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE QuasiQuotes #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+
+module Test.Control.Monad.TestFixture.DefaultSignaturesSpec (spec) where
+
+import Test.Hspec
+
+import Control.Monad.Trans.Class (MonadTrans(..))
+import Control.Monad.TestFixture.TH
+
+class Monad m => MonadFoo m where
+  foo :: m ()
+
+  default foo :: (MonadTrans t, MonadFoo m', m ~ t m') => m ()
+  foo = lift foo
+
+mkFixture "Fixture" [ts| MonadFoo |]
+
+spec :: Spec
+spec = describe "mkFixture" $
+  it "can derive classes that use DefaultSignatures" $ example $
+    return ()
diff --git a/test/Test/Control/Monad/TestFixture/THSpec.hs b/test/Test/Control/Monad/TestFixture/THSpec.hs
--- a/test/Test/Control/Monad/TestFixture/THSpec.hs
+++ b/test/Test/Control/Monad/TestFixture/THSpec.hs
@@ -18,7 +18,6 @@
 import Control.Monad.Except (runExcept, throwError)
 import Control.Monad.Fail (MonadFail(..))
 import Language.Haskell.TH.Syntax
-import Language.Haskell.TH.ToExp (toExp)
 
 import Control.Monad.TestFixture
 import Control.Monad.TestFixture.TH
@@ -35,7 +34,7 @@
       let fixture = def
             { _qReport = \b s -> when b $ throwError s
             , _qNewName = \s -> return $ Name (OccName s) (NameU 0)
-            , _qReify = \_ -> return $(toExp <$> reify ''MultiParam)
+            , _qReify = \_ -> return $(lift =<< reify ''MultiParam)
             }
       let result = runExcept $ unTestFixtureT (runQ $ mkFixture "Fixture" [ts| MultiParam |]) fixture
       result `shouldBe` (Left $