Ota updates after rooting, unlocking bootloader

Ok... So I rooted my nexus 5, and unlocked the bootloader. When I tried to install the 5.1.1 ota update, my phone went into recovery and then showed a dead Android robot and gave me ''Error!". I have heard a lot that if u modify any system file on lollipop, ota updates will fail. So then I unrooted, and then relocked the bootloader. When I tried to install the update, the same thing happened. My nexus booted into recovery and gave me Error!. I am tired of using nexus root toolkit to update to the latest software and want to use ota updates. I am selling my phone to a unexperienced person pretty soon and want it to function properly aka be able to use OTA's. Any help would be appreciated. Thanks in advance.

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