Fix a panic introduced by #1389
Fix a panic introduced by #1389

Signed-off-by: Santhosh Manohar <0806b0927a65802677e1bc58a740a42f957953d7@docker.com>

nalind/docker,cpuguy83/docker,jthelin/docker,moul/docker,jameszhan/docker,Mokolea/docker,paralin/docker,trebonian/docker,moul/docker,Ramzec/docker,jameszhan/docker,jthelin/docker,bearstech/moby,bearstech/moby,trebonian/docker,konstruktoid/docker-upstream,jimjag/docker,Ramzec/docker,nalind/docker,djs55/docker,jimjag/docker,docker/docker,AkihiroSuda/docker,Mokolea/docker,mahak/docker,vdemeester/docker,cpuguy83/docker,Mokolea/docker,AkihiroSuda/docker,djs55/docker,thaJeztah/docker,jimjag/docker,flavio/docker,moby/moby,yosifkit/docker,thaJeztah/docker,gnailuy/docker,nawawi/docker,paralin/docker,AkihiroSuda/docker,moul/docker,Ramzec/docker,nawawi/docker,AkihiroSuda/docker,trebonian/docker,thaJeztah/docker,paralin/docker,mahak/docker,moby/moby,Mokolea/docker,konstruktoid/docker-upstream,vdemeester/docker,yosifkit/docker,cpuguy83/docker,nalind/docker,jimjag/docker,mahak/docker,mahak/docker,gnailuy/docker,gnailuy/docker,trebonian/docker,moby/moby,djs55/docker,konstruktoid/docker-upstream,yosifkit/docker,nalind/docker,moul/docker,moby/moby,cpuguy83/docker,thaJeztah/docker,bearstech/moby,yosifkit/docker,docker/docker,jameszhan/docker,nawawi/docker,flavio/docker,nawawi/docker,flavio/docker,bearstech/moby,paralin/docker,gnailuy/docker,Ramzec/docker,flavio/docker,docker/docker,konstruktoid/docker-upstream,vdemeester/docker