diff --git a/sandbox/sandbox.cpp b/sandbox/sandbox.cpp
index d2d483e9e..6ec18107f 100644
--- a/sandbox/sandbox.cpp
+++ b/sandbox/sandbox.cpp
@@ -219,7 +219,7 @@ struct Everything
     ar(CEREAL_NVP(s));
   }
 
-  bool operator==(Everything const & o)
+  bool operator==(Everything const & o) const
   {
     return
       x == o.x &&