diff --git a/include/gsl/assert b/include/gsl/assert
index 7d05044..1c2d96e 100644
--- a/include/gsl/assert
+++ b/include/gsl/assert
@@ -109,11 +109,11 @@ public:
                                              return chandler.exchange(h); }
     auto get_handler()          -> handler { return chandler; }
 
-    auto expects(bool b) { assertion(b); }
-    auto ensures(bool b) { assertion(b); }
+    void expects(bool b) { assertion(b); }
+    void ensures(bool b) { assertion(b); }
 
 private:
-    auto assertion(bool b) { if (!b) chandler.load()(); }
+    void assertion(bool b) { if (!b) chandler.load()(); }
     std::atomic<handler> chandler;
 };