diff --git a/include/wia_lh.idl b/include/wia_lh.idl
index 284528a6438..2e3ecf44945 100644
--- a/include/wia_lh.idl
+++ b/include/wia_lh.idl
@@ -47,7 +47,7 @@ interface IWiaDevMgr : IUnknown
         [in] HWND hwndParent,
         [in] LONG lDeviceType,
         [in] LONG lFlags,
-        [in, out] BSTR *pbstrDeviceID,
+        [in, out, unique] BSTR *pbstrDeviceID,
         [retval, out] IWiaItem **ppItemRoot);
 
     HRESULT SelectDeviceDlgID(
diff --git a/include/wia_xp.idl b/include/wia_xp.idl
index 284528a6438..2e3ecf44945 100644
--- a/include/wia_xp.idl
+++ b/include/wia_xp.idl
@@ -47,7 +47,7 @@ interface IWiaDevMgr : IUnknown
         [in] HWND hwndParent,
         [in] LONG lDeviceType,
         [in] LONG lFlags,
-        [in, out] BSTR *pbstrDeviceID,
+        [in, out, unique] BSTR *pbstrDeviceID,
         [retval, out] IWiaItem **ppItemRoot);
 
     HRESULT SelectDeviceDlgID(