7401

diff --git a/compiler/typecheck/TcGenDeriv.lhs b/compiler/typecheck/TcGenDeriv.lhs
index 77bda82..1347f14 100644
--- a/compiler/typecheck/TcGenDeriv.lhs
+++ b/compiler/typecheck/TcGenDeriv.lhs
@@ -171,7 +171,7 @@ gen_Eq_binds loc tycon
     fall_through_eqn
       | no_tag_match_cons   -- All constructors have arguments
       = case pat_match_cons of
-          []  -> []   -- No constructors; no fall-though case
+          []  -> [([], undefined_Expr)]   -- No constructors; no fall-though case
           [_] -> []   -- One constructor; no fall-though case
           _   ->      -- Two or more constructors; add fall-through of
                       --       (==) _ _ = False
@@ -2144,7 +2144,7 @@ bs_RDRs         = [ mkVarUnqual (mkFastString ("b"++show i)) | i <- [(1::Int) ..
 cs_RDRs         = [ mkVarUnqual (mkFastString ("c"++show i)) | i <- [(1::Int) .. ] ]

 a_Expr, c_Expr, f_Expr, z_Expr, ltTag_Expr, eqTag_Expr, gtTag_Expr,
-    false_Expr, true_Expr, fmap_Expr, pure_Expr, mempty_Expr, foldMap_Expr, traverse_Expr :: LHsExpr RdrName
+    false_Expr, undefined_Expr, true_Expr, fmap_Expr, pure_Expr, mempty_Expr, foldMap_Expr, traverse_Expr :: LHsExpr RdrName
 a_Expr          = nlHsVar a_RDR
 -- b_Expr       = nlHsVar b_RDR
 c_Expr          = nlHsVar c_RDR
@@ -2154,6 +2154,7 @@ ltTag_Expr      = nlHsVar ltTag_RDR
 eqTag_Expr      = nlHsVar eqTag_RDR
 gtTag_Expr      = nlHsVar gtTag_RDR
 false_Expr      = nlHsVar false_RDR
+undefined_Expr  = nlHsVar undefined_RDR
 true_Expr       = nlHsVar true_RDR
 fmap_Expr       = nlHsVar fmap_RDR
 pure_Expr       = nlHsVar pure_RDR