diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
index 175704c6549a..5b98b6a8f7bb 100644
--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -2332,24 +2332,18 @@ nsXULPrototypeElement::Deserialize(nsIObjectInputStream* aStream,
             switch (childType) {
             case eType_Element:
                 child = new nsXULPrototypeElement();
-                child->mType = childType;
-
                 rv = child->Deserialize(aStream, aProtoDoc, aDocumentURI,
                                         aNodeInfos);
                 if (NS_WARN_IF(NS_FAILED(rv))) return rv;
                 break;
             case eType_Text:
                 child = new nsXULPrototypeText();
-                child->mType = childType;
-
                 rv = child->Deserialize(aStream, aProtoDoc, aDocumentURI,
                                         aNodeInfos);
                 if (NS_WARN_IF(NS_FAILED(rv))) return rv;
                 break;
             case eType_PI:
                 child = new nsXULPrototypePI();
-                child->mType = childType;
-
                 rv = child->Deserialize(aStream, aProtoDoc, aDocumentURI,
                                         aNodeInfos);
                 if (NS_WARN_IF(NS_FAILED(rv))) return rv;
@@ -2358,7 +2352,6 @@ nsXULPrototypeElement::Deserialize(nsIObjectInputStream* aStream,
                 // language version/options obtained during deserialization.
                 nsXULPrototypeScript* script = new nsXULPrototypeScript(0, 0);
                 child = script;
-                child->mType = childType;
 
                 rv = aStream->ReadBoolean(&script->mOutOfLine);
                 if (NS_WARN_IF(NS_FAILED(rv))) return rv;
@@ -2384,6 +2377,7 @@ nsXULPrototypeElement::Deserialize(nsIObjectInputStream* aStream,
             }
 
             MOZ_ASSERT(child, "Don't append null to mChildren");
+            MOZ_ASSERT(child->mType == childType);
             mChildren.AppendElement(child);
 
             // Oh dear. Something failed during the deserialization.