diff --git a/include/interval.hpp b/include/interval.hpp
index 6dc356c..172a851 100644
--- a/include/interval.hpp
+++ b/include/interval.hpp
@@ -402,7 +402,7 @@ constexpr bool operator!=(
     const interval<T> & t,
     const interval<U> & u
 ){
-    return ! t == u;
+    return ! (t == u);
 }
 
 template<typename T, typename U>