diff --git a/ctl/shared_ptr.h b/ctl/shared_ptr.h
index 2d10835a0..076a19d8e 100644
--- a/ctl/shared_ptr.h
+++ b/ctl/shared_ptr.h
@@ -322,7 +322,7 @@ class shared_ptr
         return rc ? rc->use_count() : 0;
     }
 
-    operator bool() const noexcept
+    explicit operator bool() const noexcept
     {
         return p;
     }