ImportError: No module named rosmaster.master_apir

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ı installed roskinetic and i use ubuntu 16.04 and python 2.7. when i run roscore in terminal it gives me this error ImportError: No module named rosmaster.master_api pls give me some solutions i think my bashr has some problems but i don't know what they are thank you

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output: Traceback (most recent call last): File "/opt/ros/kinetic/bin/roscore", line 36, in <module> from rosmaster.master_api import NUM_WORKERS ImportError: No module named rosmaster.master_api