diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index c3dc4d2311a..166ce3ab02f 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -2283,8 +2283,8 @@ class CGIDLInterface(CGThing):
         name = self.descriptor.name
         if (interface.getUserData("hasConcreteDescendant", False) or
                 interface.getUserData("hasProxyDescendant", False)):
-            depth = len(self.descriptor.prototypeChain)
-            check = "class.interface_chain[%s] == PrototypeList::ID::%s" % (depth - 1, name)
+            depth = self.descriptor.prototypeDepth
+            check = "class.interface_chain[%s] == PrototypeList::ID::%s" % (depth, name)
         elif self.descriptor.proxy:
             check = "class as *const _ == &Class as *const _"
         else:
diff --git a/components/script/dom/bindings/codegen/Configuration.py b/components/script/dom/bindings/codegen/Configuration.py
index 2106ff4adee..ac78e52c071 100644
--- a/components/script/dom/bindings/codegen/Configuration.py
+++ b/components/script/dom/bindings/codegen/Configuration.py
@@ -296,6 +296,7 @@ class Descriptor(DescriptorProvider):
         while parent:
             self.prototypeChain.insert(0, parent.identifier.name)
             parent = parent.parent
+        self.prototypeDepth = len(self.prototypeChain) - 1
         config.maxProtoChainLength = max(config.maxProtoChainLength,
                                          len(self.prototypeChain))