Call method http_headers_inject a nil value

[kakireddyp]

@rnburn

Getting below error

content_by_lua(nginx.conf:108):11: attempt to call method 'http_headers_inject' (a nil value)

Below Code:
local t = tracer:new()
local span = t:start_span("hello-opentracing")
local context = span:context()
local carrier = ngx.req.get_headers()
t:http_headers_inject(context,carrier)

Any Idea