diff --git a/modular.cabal b/modular.cabal
--- a/modular.cabal
+++ b/modular.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 8b8be5a6a4111f28f05f23a955f6eb1b223646287747327c9a378a686e96670e
+-- hash: 804311adb70c067aeb3a5d243c42fc84b25541647151c090b4425f273b520348
 
 name:           modular
-version:        0.1.0.6
+version:        0.1.0.7
 synopsis:       Type-safe modular arithmetic
 description:    Please the module documentation for Numeric.Modular.
 category:       Math
diff --git a/src/Numeric/Modular.hs b/src/Numeric/Modular.hs
--- a/src/Numeric/Modular.hs
+++ b/src/Numeric/Modular.hs
@@ -18,6 +18,13 @@
     >>> 15 + 3 :: Mod 7
     4
 
+    Attempts to perform arithmetic on different modular types result in type errors.
+
+    >>> (10 :: Mod 3) + (15 :: Mod 7)
+    (...)error:
+        • Couldn't match type ‘7’ with ‘3’
+    (...)
+
     Modular reductions are performed implicitly, so modular exponentiation can be performed efficiently.
 
     >>> 60803790666453028877 ^ 88100461154844882932 :: Mod 39127526509442054532