diff --git a/dom/base/nsDOMClassInfoID.h b/dom/base/nsDOMClassInfoID.h
index 6c5eaa88db43..9bbd4517e59d 100644
--- a/dom/base/nsDOMClassInfoID.h
+++ b/dom/base/nsDOMClassInfoID.h
@@ -73,6 +73,7 @@ enum nsDOMClassInfoID {
  *          that implements one of these interfaces must be directly castable
  *          to that interface from the *canonical* nsISupports!
  */
+#undef DOMCI_CASTABLE_INTERFACE
 #define DOMCI_CASTABLE_INTERFACES(_extra)                                     \
 DOMCI_CASTABLE_INTERFACE(nsINode, nsINode, 0, _extra)                         \
 DOMCI_CASTABLE_INTERFACE(nsIContent, nsIContent, 1, _extra)                   \
diff --git a/js/src/xpconnect/src/nsDOMQS.h b/js/src/xpconnect/src/nsDOMQS.h
index 6625916f46b0..e584d3ca817b 100644
--- a/js/src/xpconnect/src/nsDOMQS.h
+++ b/js/src/xpconnect/src/nsDOMQS.h
@@ -78,6 +78,7 @@ xpc_qsUnwrapArg<_interface>(JSContext *cx,                                    \
 
 #undef DOMCI_CASTABLE_INTERFACE
 
+#undef DOMCI_CASTABLE_INTERFACE
 #define DOMCI_CASTABLE_INTERFACE(_interface, _base, _bit, _extra)             \
   DEFINE_UNWRAP_CAST(_interface, _base, _bit)