diff --git a/dlls/amstream/multimedia.c b/dlls/amstream/multimedia.c
index 9ca38d9fa43..e62c930a399 100644
--- a/dlls/amstream/multimedia.c
+++ b/dlls/amstream/multimedia.c
@@ -244,6 +244,12 @@ static HRESULT WINAPI multimedia_stream_Initialize(IAMMultiMediaStream *iface,
 
     TRACE("mmstream %p, type %u, flags %#x, graph %p.\n", mmstream, type, flags, graph);
 
+    if (graph && mmstream->graph)
+    {
+        WARN("Graph already initialized, returning E_INVALIDARG.\n");
+        return E_INVALIDARG;
+    }
+
     if (graph && FAILED(hr = create_graph(mmstream, graph)))
         return hr;
 
diff --git a/dlls/amstream/tests/amstream.c b/dlls/amstream/tests/amstream.c
index 3555eff9704..e2c00ebc3ae 100644
--- a/dlls/amstream/tests/amstream.c
+++ b/dlls/amstream/tests/amstream.c
@@ -1646,6 +1646,15 @@ static void test_initialize(void)
     ok(hr == S_OK, "Got hr %#x.\n", hr);
     ok(!got_add_filter, "Got %d calls to IGraphBuilder::AddFilter().\n", got_add_filter);
 
+    hr = IAMMultiMediaStream_Initialize(mmstream, STREAMTYPE_READ, 0, (IGraphBuilder *)&graph);
+    ok(hr == E_INVALIDARG, "Got hr %#x.\n", hr);
+    hr = IAMMultiMediaStream_Initialize(mmstream, STREAMTYPE_READ, 0, NULL);
+    ok(hr == S_OK, "Got hr %#x.\n", hr);
+    hr = IAMMultiMediaStream_Initialize(mmstream, STREAMTYPE_WRITE, 0, NULL);
+    todo_wine ok(hr == E_INVALIDARG, "Got hr %#x.\n", hr);
+    hr = IAMMultiMediaStream_Initialize(mmstream, STREAMTYPE_TRANSFORM, 0, NULL);
+    todo_wine ok(hr == E_INVALIDARG, "Got hr %#x.\n", hr);
+
     IMediaStreamFilter_Release(filter);
     ref = IAMMultiMediaStream_Release(mmstream);
     ok(!ref, "Got outstanding refcount %d.\n", ref);