diff --git a/include/vkd3d_d3d12.idl b/include/vkd3d_d3d12.idl
index ad1738ef..a63dc32c 100644
--- a/include/vkd3d_d3d12.idl
+++ b/include/vkd3d_d3d12.idl
@@ -26,11 +26,11 @@ cpp_quote("#ifndef _D3D12_CONSTANTS")
 cpp_quote("#define _D3D12_CONSTANTS")
 
 cpp_quote("#ifndef D3D12_ERROR_ADAPTER_NOT_FOUND")
-cpp_quote("#define D3D12_ERROR_ADAPTER_NOT_FOUND 0x887e0001")
+cpp_quote("#define D3D12_ERROR_ADAPTER_NOT_FOUND ((HRESULT)0x887e0001)")
 cpp_quote("#endif")
 
 cpp_quote("#ifndef D3D12_ERROR_DRIVER_VERSION_MISMATCH")
-cpp_quote("#define D3D12_ERROR_DRIVER_VERSION_MISMATCH 0x887e0002")
+cpp_quote("#define D3D12_ERROR_DRIVER_VERSION_MISMATCH ((HRESULT)0x887e0002)")
 cpp_quote("#endif")
 
 const UINT D3D12_CS_TGSM_REGISTER_COUNT = 8192;