diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -168,7 +168,7 @@
 
 instance ToElementalComposition Ammonium where
   toElementalComposition _ = mkElementalComposition [(N, 1), (H, 4)]
-  charge _ = 1
+  charge _ = Just 1
 
 instance Ion Ammonium
 
diff --git a/isotope.cabal b/isotope.cabal
--- a/isotope.cabal
+++ b/isotope.cabal
@@ -1,5 +1,5 @@
 name:                isotope
-version:             0.5.0.0
+version:             0.5.0.1
 synopsis:            Isotopic masses and relative abundances.
 description:         Please see README.md
 homepage:            https://github.com/Michaelt293/Element-isotopes/blob/master/README.md