diff --git a/compiler/typecheck/TcDerivInfer.hs b/compiler/typecheck/TcDerivInfer.hs
index 93dcf4383ccf3b5c3b7b95b79d9bc46d26a37308..bbd054cb937f3a9ddd55207e954b8884b963a102 100644
--- a/compiler/typecheck/TcDerivInfer.hs
+++ b/compiler/typecheck/TcDerivInfer.hs
@@ -622,6 +622,8 @@ simplifyDeriv pred tvs thetas
        -- Simplify the constraints
        ; solved_implics <- runTcSDeriveds $ solveWantedsAndDrop
                                           $ unionsWC wanteds
+       -- It's not yet zonked!  Obviously zonk it before peering at it
+       ; solved_implics <- zonkWC solved_implics
 
        -- See [STEP DAC HOIST]
        -- Split the resulting constraints into bad and good constraints,
diff --git a/testsuite/tests/deriving/should_compile/T14339.hs b/testsuite/tests/deriving/should_compile/T14339.hs
new file mode 100644
index 0000000000000000000000000000000000000000..e2521f2de0c741bac0af1345f532fa0141c827f0
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T14339.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE UndecidableInstances #-}
+module Bug where
+
+import GHC.TypeLits
+
+newtype Baz = Baz Foo
+  deriving Bar
+
+newtype Foo = Foo Int
+
+class Bar a where
+  bar :: a
+
+instance (TypeError (Text "Boo")) => Bar Foo where
+  bar = undefined
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 5f94f9d7e4168ab7041deeec359b86a8452f9fd5..86648d3edf2561436b9ada294215da107d4f2b1f 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -89,3 +89,4 @@ test('T13272', normal, compile, [''])
 test('T13272a', normal, compile, [''])
 test('T13297', normal, compile, [''])
 test('T14331', normal, compile, [''])
+test('T14339', normal, compile, [''])