diff --git a/dlls/msxml3/schema.c b/dlls/msxml3/schema.c
index 7a1f8b376d8..41d16408717 100644
--- a/dlls/msxml3/schema.c
+++ b/dlls/msxml3/schema.c
@@ -1226,12 +1226,13 @@ static HRESULT WINAPI schema_cache_add(IXMLDOMSchemaCollection2* iface, BSTR uri
             break;
 
         case VT_DISPATCH:
+        case VT_UNKNOWN:
             {
                 xmlDocPtr doc = NULL;
                 cache_entry* entry;
                 CacheEntryType type;
                 IXMLDOMNode* domnode = NULL;
-                IDispatch_QueryInterface(V_DISPATCH(&var), &IID_IXMLDOMNode, (void**)&domnode);
+                IUnknown_QueryInterface(V_UNKNOWN(&var), &IID_IXMLDOMNode, (void**)&domnode);
 
                 if (domnode)
                     doc = xmlNodePtr_from_domnode(domnode, XML_DOCUMENT_NODE)->doc;
@@ -1275,10 +1276,9 @@ static HRESULT WINAPI schema_cache_add(IXMLDOMSchemaCollection2* iface, BSTR uri
             break;
 
         default:
-            {
-                heap_free(name);
-                return E_INVALIDARG;
-            }
+            FIXME("arg type is not supported, %s\n", debugstr_variant(&var));
+            heap_free(name);
+            return E_INVALIDARG;
     }
     heap_free(name);
     return S_OK;
diff --git a/dlls/msxml3/tests/schema.c b/dlls/msxml3/tests/schema.c
index 43ddc3cb851..bd5c3595d66 100644
--- a/dlls/msxml3/tests/schema.c
+++ b/dlls/msxml3/tests/schema.c
@@ -506,6 +506,7 @@ static void test_schema_refs(void)
     VARIANT_BOOL b;
     BSTR str;
     LONG len;
+    HRESULT hr;
 
     doc = create_document(&IID_IXMLDOMDocument2);
     if (!doc)
@@ -557,6 +558,24 @@ static void test_schema_refs(void)
     ole_check(IXMLDOMSchemaCollection_get_length(cache, &len));
     ok(len == 0, "got %d\n", len);
 
+    /* same, but with VT_UNKNOWN type */
+    V_VT(&v) = VT_UNKNOWN;
+    V_UNKNOWN(&v) = (IUnknown*)doc;
+    hr = IXMLDOMSchemaCollection_add(cache, NULL, v);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+
+    len = -1;
+    hr = IXMLDOMSchemaCollection_get_length(cache, &len);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    ok(len == 1, "got %d\n", len);
+
+    hr = IXMLDOMSchemaCollection_remove(cache, NULL);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    len = -1;
+    hr = IXMLDOMSchemaCollection_get_length(cache, &len);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    ok(len == 0, "got %d\n", len);
+
     str = SysAllocString(xdr_schema_uri);
     ole_check(IXMLDOMSchemaCollection_add(cache, str, _variantdoc_(doc)));