diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,7 @@
 # Changelog for the [`ghc-typelits-knownnat`](http://hackage.haskell.org/package/ghc-typelits-knownnat) package
 
+## 0.1.1 *August 11th 2016*
+* Fixes bug: panic on a non-given KnownNat constraint variable
+
 ## 0.1 *August 10th 2016*
 * Initial release
diff --git a/ghc-typelits-knownnat.cabal b/ghc-typelits-knownnat.cabal
--- a/ghc-typelits-knownnat.cabal
+++ b/ghc-typelits-knownnat.cabal
@@ -1,5 +1,5 @@
 name:                ghc-typelits-knownnat
-version:             0.1
+version:             0.1.1
 synopsis:            Derive KnownNat constraints from other KnownNat constraints
 description:
   A type checker plugin for GHC that can derive \"complex\" @KnownNat@
diff --git a/src/GHC/TypeLits/KnownNat/Solver.hs b/src/GHC/TypeLits/KnownNat/Solver.hs
--- a/src/GHC/TypeLits/KnownNat/Solver.hs
+++ b/src/GHC/TypeLits/KnownNat/Solver.hs
@@ -60,7 +60,7 @@
 import Module     (mkModuleName)
 import OccName    (mkTcOcc)
 import Outputable (Outputable (..), (<+>), integer, text, vcat)
-import Panic      (panic, pgmErrorDoc)
+import Panic      (panicDoc, pgmErrorDoc)
 import Plugins    (Plugin (..), defaultPlugin)
 import PrelNames  (knownNatClassName)
 import TcEvidence (EvTerm (..), EvLit (EvNum), mkEvCast, mkTcSymCo, mkTcTransCo)
@@ -239,16 +239,17 @@
 constraintToEvTerm defs kn_map (ct,cls,op) = (,ct) <$> go op
   where
     go (I i) = makeLitDict cls (mkNumLitTy i) i
-    go (V v) | Just (ct',_,_) <- lookup v kn_map =
-      let ct_ev = ctEvidence ct'
-          evT   = ctev_evar ct_ev
-      in  Just (EvId evT)
+    go (V v) = case lookup v kn_map of
+      Just (ct',_,_) -> let ct_ev = ctEvidence ct'
+                            evT   = ctev_evar ct_ev
+                        in  Just (EvId evT)
+      Nothing -> Nothing
     go e = do
       let (x,y,df) = case e of
             Add x' y' -> (x',y',knAddDFunId defs)
             Mul x' y' -> (x',y',knMulDFunId defs)
             Exp x' y' -> (x',y',knExpDFunId defs)
-            _ -> panic "GHC.TypeLits.KnownNat.Solver: not an op"
+            _ -> panicDoc "GHC.TypeLits.KnownNat.Solver: not an op" (ppr e)
       x' <- go x
       y' <- go y
       makeOpDict df cls (reifyOp x) (reifyOp y) (reifyOp e) x' y'
diff --git a/tests/Main.hs b/tests/Main.hs
--- a/tests/Main.hs
+++ b/tests/Main.hs
@@ -31,7 +31,7 @@
     [ testCase "KnownNat 4 + KnownNat 6 ~ 10" $
       show (test1 (Proxy @ 4)) @?=
       "10"
-    , testCase "KnownNat 4 + KnownNat 3 ~ 12" $
+    , testCase "KnownNat 4 * KnownNat 3 ~ 12" $
       show (test2 (Proxy @ 4)) @?=
       "12"
     , testCase "KnownNat 2 + KnownNat 7 ~ 9" $