Failed to lock driver mip-map surface   

Dear All,

My code is effectively:

IDirect3DTexture9 *imageTexture;
D3DLOCKED_RECT lr;

if (SUCCEEDED( d3dDevice->CreateTexture(748, 942, 1, D3DUSAGE_DYNAMIC, 
D3DFMT_X8R8G8B8, D3DPOOL_DEFAULT, &imageTexture, NULL) ) &&
    SUCCEEDED( imageTexture->LockRect(0, &lr, NULL, 0)  ))
    ...


What could cause the "Failed to lock driver mip-map surface" error that 
LockRect is returning?

My application can open many D3D windows.In order to prevent "out of video 
memory" errors I create the backbuffer each time a repaint is required and 
set it to 1x1 afterwards.
I can successfully open six windows then the crash occurs in the paint 
handler of the seventh.
Could calling changing the backbuffer with IDirect3DDevice9::Reset be the 
cause?

I am running the application using the debug libraries and compiled with the 
D3D_DEBUG_INFO symbol and this is the first error that is reported.

Regards,
Andrew

date: Tue, 2 Oct 2007 14:22:36 +0100   author:   Andrew Jarvis someone@somewhere