Fragment SQL injection warning

gistfile1.txt

iex(3)> where_string = "device_id = AAA OR login_id = 123"
"device_id = AAA OR login_id = 123"
iex(4)> query = from u in Portmeirion.User, limit: 1, select: %{warehouse_id: u.warehouse_id}, where: fragment(^where_string)
** (ArgumentError) to prevent sql injection, only a keyword list may be interpolated as the first argument to `fragment/1` with the `^` operator, got `"device_id = AAA OR login_id = 123"`
    (ecto) lib/ecto/query/builder.ex:512: Ecto.Query.Builder.keyword!/1