prometheus-to-sd does not work

I'm using v0.5.2 with this output in prometheus text format

`# HELP http_server_requests_uri_root_1_min_request_rate_total App 1 minute request rate`
`# TYPE http_server_requests_uri_root_1_min_request_rate_total counter`
`http_server_requests_uri_root_1_min_request_rate_total 77.0`

And stackdriver log shows:

I Taking source configs from flags
I Taking source configs from kubernetes api server
I Built the following source configs: [{webapp localhost 8080 /manage/prometheus [] 0xc42000e1e0}]
I Running prometheus-to-sd, monitored target is webapp localhost:8080
E Could not build time series for component webapp: text format parsing error in line 1: invalid metric name
E Could not build time series for component webapp: text format parsing error in line 1: invalid metric name

Pod configuration:
- /monitor
- --source=webapp:http://localhost:8080/manage/prometheus
- --stackdriver-prefix=custom.googleapis.com
- --pod-id=$(POD_ID)
- --namespace-id=$(POD_NAMESPACE)