Prevent KeyError when popping modified field from readed value

issue3349
review1052003
(transplanted from 3cbb5dc3f8e88bb826aec28053eefc99b56d2ef2)

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extra : transplant_source : %3C%BB%5D%C3%F8%E8%8B%B8%26%AE%C2%80S%EE%FC%99%B5m.%F2