How do I get OpenGL errors?

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ImaginaryHuman

I open a valid display, and initialize all the OpenGL view ready to start drawing.

Then I do this:

repeat
until glGetError()=0  'wipe existing errors
glCopyPixels(0,0,8,8,GL_STENCIL)
If glGetError()>0 Then Print "Error copying in stencil buffer - no stencil buffer" Else Print "No error"

However, this always shows "No error", regardless of whether I have a stencil buffer available or not (as set up with flags in CreateGraphics().

How can I get the error? According to the OpenGL blue book, "GL_INVALID_OPERATION is generated if type is GL_STENCIL and there is no stencil buffer."

But I always get GL_NO_ERROR.

?????????