diff --git a/include/gsl.h b/include/gsl.h
index c9fb2d2..b1543bf 100644
--- a/include/gsl.h
+++ b/include/gsl.h
@@ -120,7 +120,7 @@ public:
     }
 
     template <typename U, typename Dummy = std::enable_if_t<std::is_convertible<U, T>::value>>
-    bool operator != (const not_null<U>& rhs) const
+    bool operator!=(const not_null<U>& rhs) const
     {
         return !(*this == rhs.get());
     }