Couldn't serialize object of type "NumericIncrementTransform" (found in field "count")

[vhanded]

I am having this issue when calling fulltextsearch.set()

Error: Value for argument "data" is not a valid Firestore document. Couldn't serialize object of type "NumericIncrementTransform" (found in field "count"). Firestore doesn't support JavaScript objects with custom prototypes (i.e. objects that were created via the "new" operator).
    at validateUserInput (/home/vhanded/Development/flow/functions/node_modules/@google-cloud/firestore/build/src/serializer.js:311:15)
    at Object.validateUserInput (/home/vhanded/Development/flow/functions/node_modules/@google-cloud/firestore/build/src/serializer.js:260:13)
    at validateDocumentData (/home/vhanded/Development/flow/functions/node_modules/@google-cloud/firestore/build/src/write-batch.js:641:18)
    at WriteBatch.set (/home/vhanded/Development/flow/functions/node_modules/@google-cloud/firestore/build/src/write-batch.js:251:9)
    at WriteBatch2.commit (/home/vhanded/Development/flow/node_modules/firestore-full-text-search/lib/utils/firestore.js:68:38)
    at FirestoreFullTextSearch.set (/home/vhanded/Development/flow/node_modules/firestore-full-text-search/lib/index.js:205:21)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async /home/vhanded/Development/flow/functions/lib/customerEvents.js:64:9
    at async runFunction (/home/vhanded/.nvm/versions/node/v16.3.0/lib/node_modules/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:583:9)
    at async runBackground (/home/vhanded/.nvm/versions/node/v16.3.0/lib/node_modules/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:595:5)
    at async processBackground (/home/vhanded/.nvm/versions/node/v16.3.0/lib/node_modules/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:578:5)
    at async invokeTrigger (/home/vhanded/.nvm/versions/node/v16.3.0/lib/node_modules/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:653:13)
    at async handleMessage (/home/vhanded/.nvm/versions/node/v16.3.0/lib/node_modules/firebase-tools/lib/emulator/functionsEmulatorRuntime.js:741:9)

This is the environment I am using:

Angular CLI: 12.0.5
Node: 16.3.0
Package Manager: npm 7.15.1
OS: linux x64

Angular: 12.0.5
... animations, cdk, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router, service-worker

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1200.5
@angular-devkit/build-angular   12.0.5
@angular-devkit/core            12.0.5
@angular-devkit/schematics      12.0.5 (cli-only)
@angular/fire                   6.1.5
@ngtools/webpack                12.0.5
@schematics/angular             12.0.5 (cli-only)
rxjs                            6.6.7
typescript                      4.2.4

I don't have 'count' field in my data, and I have no idea why this is happening.