Adding columns with map where new name is a variable fails

[DrChainsaw]

dff = DataFrame(a=1:3, b=3:5)

#No problem with litterals:
addcol(row) = merge(row, (;:c => 1))

dff |> @map(addcol(_)) |> DataFrame

# But this fails:
addcol(row, cname) = merge(row, (;cname => 1))

dff |> @map(addcol(_, :c)) |> DataFrame
ERROR: MethodError: no method matching nondatavaluenamedtuple(::Type{NamedTuple})
Closest candidates are:
  nondatavaluenamedtuple(::Type{NT}) where {names, NT<:(NamedTuple{names,T} where T<:Tuple)} at C:\Users\echrska\.julia\packages\Tables\okt7x\src\tofromdatavalues.jl:2
Stacktrace:
 [1] schema(::Tables.IteratorWrapper{QueryOperators.EnumerableMap{NamedTuple,QueryOperators.EnumerableIterable{NamedTuple{(:a,
:b),Tuple{Int64,Int64}},Tables.DataValueRowIterator{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.Schema{(:a, :b),Tuple{Int64,Int64}},Tables.RowIterator{NamedTuple{(:a, :b),Tuple{Array{Int64,1},Array{Int64,1}}}}}},var"#290#292"}}) at C:\Users\echrska\.julia\packages\Tables\okt7x\src\tofromdatavalues.jl:32
 [2] columns at C:\Users\echrska\.julia\packages\Tables\okt7x\src\fallbacks.jl:237 [inlined]
 [3] DataFrame(::QueryOperators.EnumerableMap{NamedTuple,QueryOperators.EnumerableIterable{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.DataValueRowIterator{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.Schema{(:a, :b),Tuple{Int64,Int64}},Tables.RowIterator{NamedTuple{(:a, :b),Tuple{Array{Int64,1},Array{Int64,1}}}}}},var"#290#292"}; copycols::Bool) at C:\Users\echrska\.julia\packages\DataFrames\S3ZFo\src\other\tables.jl:40
 [4] DataFrame(::QueryOperators.EnumerableMap{NamedTuple,QueryOperators.EnumerableIterable{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.DataValueRowIterator{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.Schema{(:a, :b),Tuple{Int64,Int64}},Tables.RowIterator{NamedTuple{(:a, :b),Tuple{Array{Int64,1},Array{Int64,1}}}}}},var"#290#292"}) at C:\Users\echrska\.julia\packages\DataFrames\S3ZFo\src\other\tables.jl:31
 [5] |>(::QueryOperators.EnumerableMap{NamedTuple,QueryOperators.EnumerableIterable{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.DataValueRowIterator{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.Schema{(:a, :b),Tuple{Int64,Int64}},Tables.RowIterator{NamedTuple{(:a, :b),Tuple{Array{Int64,1},Array{Int64,1}}}}}},var"#290#292"}, ::Type{T} where T) at .\operators.jl:823
 [6] top-level scope at none:0

# This too:
@from r in dff begin
       @select addcol(r, :c)
       @collect DataFrame
       end
ERROR: MethodError: no method matching nondatavaluenamedtuple(::Type{NamedTuple})
Closest candidates are:
  nondatavaluenamedtuple(::Type{NT}) where {names, NT<:(NamedTuple{names,T} where T<:Tuple)} at C:\Users\echrska\.julia\packages\Tables\okt7x\src\tofromdatavalues.jl:2
Stacktrace:
 [1] schema(::Tables.IteratorWrapper{QueryOperators.EnumerableMap{NamedTuple,QueryOperators.EnumerableIterable{NamedTuple{(:a,
:b),Tuple{Int64,Int64}},Tables.DataValueRowIterator{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.Schema{(:a, :b),Tuple{Int64,Int64}},Tables.RowIterator{NamedTuple{(:a, :b),Tuple{Array{Int64,1},Array{Int64,1}}}}}},var"#297#298"}}) at C:\Users\echrska\.julia\packages\Tables\okt7x\src\tofromdatavalues.jl:32
 [2] columns at C:\Users\echrska\.julia\packages\Tables\okt7x\src\fallbacks.jl:237 [inlined]
 [3] DataFrame(::QueryOperators.EnumerableMap{NamedTuple,QueryOperators.EnumerableIterable{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.DataValueRowIterator{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.Schema{(:a, :b),Tuple{Int64,Int64}},Tables.RowIterator{NamedTuple{(:a, :b),Tuple{Array{Int64,1},Array{Int64,1}}}}}},var"#297#298"}; copycols::Bool) at C:\Users\echrska\.julia\packages\DataFrames\S3ZFo\src\other\tables.jl:40
 [4] DataFrame(::QueryOperators.EnumerableMap{NamedTuple,QueryOperators.EnumerableIterable{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.DataValueRowIterator{NamedTuple{(:a, :b),Tuple{Int64,Int64}},Tables.Schema{(:a, :b),Tuple{Int64,Int64}},Tables.RowIterator{NamedTuple{(:a, :b),Tuple{Array{Int64,1},Array{Int64,1}}}}}},var"#297#298"}) at C:\Users\echrska\.julia\packages\DataFrames\S3ZFo\src\other\tables.jl:31
 [5] top-level scope at none:0

# But this works:
DataFrame(@from r in dff begin
       @select addcol(r, :c)
       @collect
       end)