diff --git a/components/script/dom/bindings/principals.rs b/components/script/dom/bindings/principals.rs
index d693c52d01e..62ef578b1ae 100644
--- a/components/script/dom/bindings/principals.rs
+++ b/components/script/dom/bindings/principals.rs
@@ -138,6 +138,7 @@ pub unsafe extern "C" fn subsumes(obj: *mut JSPrincipals, other: *mut JSPrincipa
         let other_origin = other.origin();
         obj_origin.same_origin_domain(&other_origin)
     } else {
+        warn!("Received null JSPrincipals asrgument.")
         false
     }
 }