RenderManForBlender: Renderman 21 sudden error

Hello, I decided to try out Renderman and it has been working out fine until I got this error in a particular scene everytime I use IPR.

`Traceback (most recent call last): File “C:\Users\Shaddie\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\addons\PRMan-for-Blender-21_2\operators.py”, line 406, in invoke engine.ipr.start_interactive() File “C:\Users\Shaddie\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\addons\PRMan-for-Blender-21_2\engine.py”, line 620, in start_interactive write_rib(self, self.scene, self.ri, visible_objects) File “C:\Users\Shaddie\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\addons\PRMan-for-Blender-21_2\export.py”, line 3180, in write_rib data_blocks, instances = cache_motion(scene, rpass) File “C:\Users\Shaddie\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\addons\PRMan-for-Blender-21_2\export.py”, line 1982, in cache_motion get_instances_and_blocks(objects, rpass) File “C:\Users\Shaddie\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\addons\PRMan-for-Blender-21_2\export.py”, line 1769, in get_instances_and_blocks inst_data_blocks = get_data_blocks_needed(ob, rpass, mb_on) File “C:\Users\Shaddie\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\addons\PRMan-for-Blender-21_2\export.py”, line 1885, in get_data_blocks_needed 1].material] if psys.settings.material and len(ob.material_slots) else [] IndexError: bpy_prop_collection[index]: index 1 out of range, size 1

location: <unknown location>:-1

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then if I try to stop IPR

`Traceback (most recent call last): File “C:\Users\Shaddie\AppData\Roaming\Blender Foundation\Blender\2.78\scripts\addons\PRMan-for-Blender-21_2\operators.py”, line 415, in invoke engine.ipr.issue_transform_edits) ValueError: list.remove(x): x not in list

location: <unknown location>:-1

`

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is it possible to identify what is causing the error and how do I prevent this in the future? PS I don’t know much about coding.

`