I am getting an error 'pack with invalid duration' using ffmpeg with concat on Yosemite.

I am executing ffmpeg -i concat:"intro.mp4|movie.mp4" -codec copy output.mp4 and am getting error: [mp4 @ 0x7fc5f984ae00] Packet with invalid duration -1975 in stream 1

I can't seem to figure out what the problem is. The files are both H.264 AAC.

Can someone explain what causes this error?

Thanks!