diff --git a/src/primitives/block.h b/src/primitives/block.h
index a189592539..4f2ed36b46 100644
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -162,7 +162,7 @@ struct CBlockLocator
         vHave.clear();
     }
 
-    bool IsNull()
+    bool IsNull() const
     {
         return vHave.empty();
     }