GET_HEADER(Proxy-Authorization) get nothing

I try to get sip_header with proxy-authorization parameter

exten => _X.,1,Set(SIP_Authorization=${SIP_HEADER(Proxy-Authorization)})

buy I get empty “SIP_Authorization” variable.

When I set parameter “Authorization” I get this fields of sip_header

exten => _X.,1,Set(SIP_Authorization=${SIP_HEADER(Authorization)})

How can I get variable “SIP_Authorization” with “Proxy-Authorization” field of sip_header?