go-couchbase upr panic in UprCloseStream

In XDCR system test, we observe the following panic in the log.

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x8 pc=0x6292f4]
goroutine 2735 [running]:
runtime.panic(0x8b76a0, 0xbcf213)
/usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
github.com/couchbase/go-couchbase.(*UprFeed).UprCloseStream(0x0, 0xc29df302c0, 0x0, 0x0)
/home/couchbase/jenkins/workspace/sherlock-unix/godeps/src/github.com/couchbase/go-couchbase/upr.go:162 +0x44
github.com/couchbase/goxdcr/parts.(*DcpNozzle).closeUprStreams(0xc20829a120, 0x0, 0x0)
/home/couchbase/jenkins/workspace/sherlock-unix/goproj/src/github.com/couchbase/goxdcr/parts/dcp_nozzle.go:214 +0x280
github.com/couchbase/goxdcr/parts.(*DcpNozzle).Stop(0xc20829a120, 0x0, 0x0)
/home/couchbase/jenkins/workspace/sherlock-unix/goproj/src/github.com/couchbase/goxdcr/parts/dcp_nozzle.go:192 +0x1ab
github.com/couchbase/goxdcr/common.Part.Stop·fm(0x0, 0x0)
/home/couchbase/jenkins/workspace/sherlock-unix/goproj/src/github.com/couchbase/goxdcr/pipeline/generic_pipeline.go:250 +0x4a
github.com/couchbase/goxdcr/pipeline_utils.func·001(0xc20f115b90)
/home/couchbase/jenkins/workspace/sherlock-unix/goproj/src/github.com/couchbase/goxdcr/pipeline_utils/pipeline_utils.go:23 +0x59
created by github.com/couchbase/goxdcr/pipeline_utils.ExecWithTimeout
/home/couchbase/jenkins/workspace/sherlock-unix/goproj/src/github.com/couchbase/goxdcr/pipeline_utils/pipeline_utils.go:25 +0xff