Mistakes happened when i migrates docs from one table to another.
source doc like {"A":[], "B":"name"}
result doc like {"B":"name"}
that A field was unset
when param "a" of func "normalizeDocSlice" was a zero length slice that not nil pointer. So the returned var "avs" was a nil.

i guess var avs = make([]interface{}, 0) may solve my problem