# `Plug.Conn.InvalidHeaderError`
[🔗](https://github.com/elixir-plug/plug/blob/v1.20.1/lib/plug/conn.ex#L272)

Error raised when trying to send a header that has errors, for example:

  * the header key contains uppercase chars
  * the header value contains newlines \n

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*Consult [api-reference.md](api-reference.md) for complete listing*