diff --git a/lib/System/Console/MultiArg/Prim.hs b/lib/System/Console/MultiArg/Prim.hs
--- a/lib/System/Console/MultiArg/Prim.hs
+++ b/lib/System/Console/MultiArg/Prim.hs
@@ -472,7 +472,7 @@
 
 
 assert :: e -> Bool -> Either e ()
-assert e b = if b then Left e else Right ()
+assert e b = if b then Right () else Left e
 
 fromMaybe :: e -> Maybe a -> Either e a
 fromMaybe e = maybe (Left e) Right
diff --git a/multiarg.cabal b/multiarg.cabal
--- a/multiarg.cabal
+++ b/multiarg.cabal
@@ -1,5 +1,5 @@
 Name: multiarg
-Version: 0.20.0.0
+Version: 0.22.0.0
 Cabal-version: >=1.8
 Build-Type: Simple
 License: BSD3