diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # Revision history for inspection-testing
 
+## 0.4.2.1 -- 2019-06-07
+
+* Bugfix release
+
 ## 0.4.2 -- 2019-06-05
 
 * Be less picky if mutually recursive definitions appear in a different order
diff --git a/examples/Regression.hs b/examples/Regression.hs
new file mode 100644
--- /dev/null
+++ b/examples/Regression.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE TemplateHaskell #-}
+
+-- This is a kitchen sink test file for various regressions
+-- reported via GitHub
+
+import Test.Inspection
+
+
+-- https://github.com/nomeata/inspection-testing/issues/35
+empty1, empty2 :: [a]
+empty1 = map id []
+empty2 = []
+
+inspect $ 'empty1 === 'empty2
+
+main :: IO ()
+main = return ()
diff --git a/inspection-testing.cabal b/inspection-testing.cabal
--- a/inspection-testing.cabal
+++ b/inspection-testing.cabal
@@ -1,5 +1,5 @@
 name:                inspection-testing
-version:             0.4.2
+version:             0.4.2.1
 synopsis:            GHC plugin to do inspection testing
 description:         Some carefully crafted libraries make promises to their
                      users beyond functionality and performance.
@@ -173,3 +173,13 @@
    build-depends:       base >=4.9 && <4.13
    default-language:    Haskell2010
    ghc-options:         -main-is MutualRecursion
+
+test-suite regression
+  type:                exitcode-stdio-1.0
+  hs-source-dirs:      examples
+  main-is:             Regression.hs
+  build-depends:       inspection-testing
+  build-depends:       base >=4.9 && <4.13
+  default-language:    Haskell2010
+  if impl(ghc < 8.4)
+      ghc-options:       -fplugin=Test.Inspection.Plugin
diff --git a/src/Test/Inspection/Core.hs b/src/Test/Inspection/Core.hs
--- a/src/Test/Inspection/Core.hs
+++ b/src/Test/Inspection/Core.hs
@@ -118,9 +118,11 @@
     equate x y
         | Just e1 <- lookup x slice1
         , Just x' <- essentiallyVar e1
+        , x' `elem` map fst slice1
         = lift $ modify (S.insert (x',y))
         | Just e2 <- lookup y slice2
         , Just y' <- essentiallyVar e2
+        , y' `elem` map fst slice2
         = lift $ modify (S.insert (x,y'))
         | Just e1 <- lookup x slice1
         , Just e2 <- lookup y slice2