TypeError: Cannot read property '_id' of undefined

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View details in Rollbar: https://rollbar.com/freezy/server/items/40/

TypeError: Cannot read property '_id' of undefined
  File "/var/www/server/production/v3.1.2/src/app/files/file.blockmatch.api.ts", line 88, in version.files.forEach.f
    					releases.set((f._file as FileDocument)._id.toString(), rls);
  File "<anonymous>", line unknown, in CoreMongooseArray.forEach
  File "/var/www/server/production/v3.1.2/src/app/files/file.blockmatch.api.ts", line 87, in rls.versions.forEach.version
    				version.files.forEach(f => {
  File "<anonymous>", line unknown, in CoreMongooseArray.forEach
  File "/var/www/server/production/v3.1.2/src/app/files/file.blockmatch.api.ts", line 86, in matchedReleases.forEach.rls
    			rls.versions.forEach(version => {
  File "<anonymous>", line unknown, in Array.forEach
  File "/var/www/server/production/v3.1.2/src/app/files/file.blockmatch.api.ts", line 85, in FileBlockmatchApi.blockmatch
    		matchedReleases.forEach(rls => {
  File "<anonymous>", line unknown, in <unknown>
  File "internal/process/next_tick.js", line 229, in process._tickDomainCallback [as _tickCallback]