SRID being treated as a binary

[murraybryant]

defmacro st_transform(wkt, srid) do
quote do: expr(fragment("ST_Transform(?,?)", unquote(wkt), unquote(srid)))
end

give me the error

(DBConnection.EncodeError) Postgrex expected a binary, got 28355.

where 28355 is the srid

if i use my own fragment

fragment(
"ST_Within(?, ST_Transform(boundary, ?::integer)

it works