Starting a new test run before the previous one completes crashes karma

[mattlewis92]

14 12 2016 22:05:20.452:ERROR [karma]: TypeError: Cannot read property 'get' of null
    at /Users/mattlewis/Code/open-source/angular-calendar/node_modules/karma-remap-istanbul/index.js:42:35
    at Array.forEach (native)
    at Collection.forEach (/Users/mattlewis/Code/open-source/angular-calendar/node_modules/karma/lib/browser_collection.js:93:21)
    at /Users/mattlewis/Code/open-source/angular-calendar/node_modules/karma-remap-istanbul/index.js:41:16
    at KarmaRemapIstanbul.onRunComplete (/Users/mattlewis/Code/open-source/angular-calendar/node_modules/karma-remap-istanbul/index.js:52:7)
    at Server.<anonymous> (/Users/mattlewis/Code/open-source/angular-calendar/node_modules/karma/lib/events.js:13:22)
    at emitTwo (events.js:111:20)
    at Server.emit (events.js:191:7)
    at Timeout._onTimeout (/Users/mattlewis/Code/open-source/angular-calendar/node_modules/karma/lib/executor.js:51:17)
    at ontimeout (timers.js:365:14)
    at tryOnTimeout (timers.js:237:5)
    at Timer.listOnTimeout (timers.js:207:5)

It's caused by the coverageMap getting cleared when the new run starts