Question about Qiime assign taxonomy result

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Hi all

I try to analyze 16s amplicon data by qiime in this day, and the assign_taxonomy.py result show empty rank in many different OTU

5989 k__Bacteria;p__GN04;c__;o__;f__;g__;s__
5988 k__Archaea;p__[Parvarchaeota];c__[Parvarchaea];o__YLA114;f__;g__;s__

I try assign taxonomy in different method(rdp,uclust...) but output same result.

Is this normal? And what reason cause that?

Thanks a lot!!

WT