Raised by the HPACK decoder when a literal header field name on the wire contains an uppercase ASCII byte (0x41..0x5A). RFC 7540 §8.1.2 requires all HTTP/2 header field names to be lowercase prior to encoding; a request that violates this MUST be treated as malformed (a stream-level PROTOCOL_ERROR). The HPACK layer surfaces it as a distinct exception so the HTTP/2 layer can distinguish it from a generic compression error and respond with RST_STREAM rather than GOAWAY(COMPRESSION_ERROR).