Problem with an external mic and T2i

Hi All,

I recently bought a Zoom H1 external mic. On its own, the H1 works fine, recording and playing back in stereo as it should. Likewise, the on-board mic on the T2i works fine.

When I connect the mic to the camera via a stereo cable, the sound only plays back out of one headphone. I'm trying to avoid syncing in post production if possible.

I can't figure out what I'm missing. Is there a setting I need to change?

Thanks in advance for any suggestions!