vb6/d3d8 Optional Texture

Hi, i have a little sub that can take a Texture as a parameter... Public Sub Render(DxDev As Direct3DDevice8, Optional dTexture As Direct3DTexture8, Optional StartSegment As Long, Optional EndSegment As Long) The reason i''m doing that is that if i dont pass a texture, i dont want the device to switch texture(for optimization you know) im able to test the StartSegement parameter because it is a long,but since dTexture is a Direct3DTexture8, i cannot test Len(dTexture) or IsObject(dTexture)... So how would I test if the dTexture parameter as been used? Thanks.