git pull report "fatal: the remote end hung up unexpectedly" and "Couldn't find remote ref"

version: "GitLab Enterprise Edition 12.7.0-pre"

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when git pull from existing branch:

git pull origin pqn1256
From gitlab.com:ccnupq/test
branch pqn1256 -> FETCH_HEAD
Already up-to-date.

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when git pull from nonexistent branch

git pull origin pqn1256test
fatal: Couldn't find remote ref pqn1256test
fatal: the remote end hung up unexpectedly

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Is fatal error bug?