cylinder collision geometry results in error when combining URDFs

[robin-gdwl]

I am using pybullet. I want to combine two URDF files programmatically, to attach a tool to a robot (p.createConstraint() is not rigid enough). The example file https://github.com/bulletphysics/bullet3/blob/master/examples/pybullet/gym/pybullet_utils/examples/combineUrdf.py works without issue. However, I have a URDF that uses primitive shapes like cylinders for collision detection like so:

   <collision>
      <geometry>
        <cylinder radius="0.0515" length="0.1"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
    </collision>

It appears as though the URDF editor has an issue when converting cylinders as collision geometry. This is the result after combining the urdfs, notice the "unknown_file":

                 <collision>
			<origin rpy="0.00000 -0.00000 0.00000" xyz="0.00000 0.00000 0.00000"/>
			<geometry>
				<mesh filename="unknown_file" scale="1.00000 1.00000 1.00000"/>
			</geometry>
		</collision>

There seem to be no issues when using cylinders as visual geometry. Spheres also get converted correctly for both visual and collision geometry. Is there a mistake on my part or in the original URDF?

Thank you very much for any help or hints.