Confusing error with mixed-type inputs

I found a strange behaviour with mixed-type inputs. 2D arrays work correctly:

[(a=1,) (a='a',)] |> @filter(isa(_.a, Char)) |> collect

1-element Array{NamedTuple{(:a,),T} where T<:Tuple,1}:
 (a = 'a',)

However, 1D do not:

[(a=1,) (a='a',)][:] |> @filter(isa(_.a, Char)) |> collect

Schemaless sources cannot be passed to datavaluerows.

Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] datavaluerows(::Array{NamedTuple{(:a,),T} where T<:Tuple,1}) at /users/aplavin/.julia/packages/Tables/NEFJq/src/tofromdatavalues.jl:72
 [3] getiterator(::Array{NamedTuple{(:a,),T} where T<:Tuple,1}) at /users/aplavin/.julia/packages/Tables/NEFJq/src/Tables.jl:189
 [4] query(::Array{NamedTuple{(:a,),T} where T<:Tuple,1}) at /users/aplavin/.julia/packages/QueryOperators/PfROc/src/source_iterable.jl:7
 [5] (::getfield(Main, Symbol("##134#136")))(::Array{NamedTuple{(:a,),T} where T<:Tuple,1}) at /users/aplavin/.julia/packages/Query/UeHfO/src/standalone_query_macros.jl:221
 [6] |>(::Array{NamedTuple{(:a,),T} where T<:Tuple,1}, ::getfield(Main, Symbol("##134#136"))) at ./operators.jl:813
 [7] top-level scope at In[37]:1

Reproduce the two examples in the issue, then trace the 1D case through query, getiterator, and datavaluerows using the stacktrace. Compare it with the working 2D input and verify that filtering and collecting the mixed-type 1D NamedTuple array no longer raises the schemaless-source error.