defend against nils
defend against nils

Fixes #2508. Hopefully.

weaveworks/scope,kinvolk/scope,weaveworks/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,alban/scope,kinvolk/scope,alban/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,alban/scope,alban/scope,alban/scope,alban/scope,kinvolk/scope