I'm using the URDF converted from the XACRO file of the same name (using rosrun xacro xacro.py nao_robot_v4.urdf.xacro > nao_robot_v4.urdf) from the nao_description package. check_urdf returns no errors, but sudo gzsdf print returns the following (on another note, gzsdf segfaults if it isn't run with sudo):

 *** Error in `gzsdf': free(): invalid pointer: 0x00000000010f8ca0 ***

Just to be sure, I ran the same command with another urdf converted from an xacro file in the rrbot_description package, which converted successfully.