I'm trying to make instanced geometry in Direct3D11, and the ID3D11DeviceContext1->Map() call is failing with the very helpful error of "Invalid Parameter" when I'm attempting to update the instance buffer.

The buffer is declared as a member variable:

Microsoft::WRL::ComPtr<ID3D11Buffer> m_instanceBuffer;

Then I create it (which succeeds):

D3D11_BUFFER_DESC instanceDesc;
ZeroMemory(&instanceDesc, sizeof(D3D11_BUFFER_DESC));
instanceDesc.Usage = D3D11_USAGE_DYNAMIC;
instanceDesc.ByteWidth = sizeof(InstanceData) * MAX_INSTANCE_COUNT;
instanceDesc.BindFlags = D3D11_BIND_VERTEX_BUFFER;
instanceDesc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE;
instanceDesc.MiscFlags = 0;
instanceDesc.StructureByteStride = 0;

DX::ThrowIfFailed(d3dDevice->CreateBuffer(&instanceDesc, NULL, &m_instanceBuffer));

However, when I try to map it:

D3D11_MAPPED_SUBRESOURCE inst;
DX::ThrowIfFailed(d3dContext->Map(m_instanceBuffer.Get(), 0, D3D11_MAP_WRITE, 0, &inst));

The map call fails with E_INVALIDARG. Nothing is NULL incorrectly, and this being one of my first D3D apps I'm currently stumped on what to do next to track it down. I'm thinking I must be creating the buffer incorrectly, but I can't see how. Any input would be appreciated.