Problem exporting a clip

Hey, having a problem exporting a clip with S4Studio, does anyone know what it could be?
The Sims 4 Studio - Version 3.1.5.8
System.AggregateException: One or more errors occurred. ---> S4Studio.Blender.BlenderException:
File "<string>", line 1, in <module>
File "<string>", line 44, in <module>
File "C:\Users\[UserName]\AppData\Roaming\Blender Foundation\Blender\2.70\scripts\addons\s4studio\animation\blender.py", line 139, in load_clip
animate_bone(skeleton.pose.bones[ROOT_BONE])
KeyError: 'bpy_prop_collection[key]: key "b__ROOT__" not found'


at S4Studio.Blender.BlenderUtilities.RunBlender(String script, String blend, Boolean background, Boolean check_version)
at S4Studio.Blender.BlenderUtilities.ExportClip(String custom_blend, String custom_clip, String mannequin_folder, String mannequin_texture, String rig_resource)
at S4Studio.ViewModels.Animation.AnimationPackCustomContent.<>c__DisplayClass152_0.<DoExport>b__0()
at System.Threading.Tasks.Task.InnerInvoke()
at System.Threading.Tasks.Task.Execute()
--- End of inner exception stack trace ---
---> (Inner Exception #0) S4Studio.Blender.BlenderException:
File "<string>", line 1, in <module>
File "<string>", line 44, in <module>
File "C:\Users\[UserName]\AppData\Roaming\Blender Foundation\Blender\2.70\scripts\addons\s4studio\animation\blender.py", line 139, in load_clip
animate_bone(skeleton.pose.bones[ROOT_BONE])
KeyError: 'bpy_prop_collection[key]: key "b__ROOT__" not found'


at S4Studio.Blender.BlenderUtilities.RunBlender(String script, String blend, Boolean background, Boolean check_version)
at S4Studio.Blender.BlenderUtilities.ExportClip(String custom_blend, String custom_clip, String mannequin_folder, String mannequin_texture, String rig_resource)
at S4Studio.ViewModels.Animation.AnimationPackCustomContent.<>c__DisplayClass152_0.<DoExport>b__0()
at System.Threading.Tasks.Task.InnerInvoke()
at System.Threading.Tasks.Task.Execute()<---