Wraping the ofTexture grabbed by the ofxSyphon

I am trying to wrap ofxSyphon texture to cvMat, that is my process:

tex = client.getTexture();
tex.readToPixels(pix);
syphonMat = toCv(pix);

but there are the error while I run this line: tex.readToPixels(pix);
[ error ] ofPixels: format doesn’t support channels
[ error ] ofGLUtils: ofGetGLFormatFromPixelFormat(): unknown OF pixel format-1, returning GL_LUMINANCE

Anyone have the ideas?