diff --git a/src/request-base.js b/src/request-base.js
index cd6020bb..a2649979 100644
--- a/src/request-base.js
+++ b/src/request-base.js
@@ -662,7 +662,8 @@ RequestBase.prototype.send = function (data) {
   // merge
   if (isObject_ && isObject(this._data)) {
     for (const key in data) {
-      if (typeof data[key] == "bigint") throw new Error("Cannot serialize BigInt value to json");
+      if (typeof data[key] == 'bigint' && !data[key].toJSON)
+        throw new Error('Cannot serialize BigInt value to json');
       if (hasOwn(data, key)) this._data[key] = data[key];
     }
   }
diff --git a/test/json.js b/test/json.js
index 3d10d7b4..967c3904 100644
--- a/test/json.js
+++ b/test/json.js
@@ -132,6 +132,32 @@ describe('req.send(Object) as "json"', function () {
     done();
   });
 
+  describe('when BigInts have a .toJSON property', function () {
+    before(function () {
+      // eslint-disable-next-line node/no-unsupported-features/es-builtins
+      BigInt.prototype.toJSON = function () {
+        return this.toString();
+      };
+    });
+
+    it('should accept BigInt properties', (done) => {
+      request
+        .post(`${uri}/echo`)
+        .send({ number: 1n })
+        .end((error, res) => {
+          res.should.be.json();
+          res.text.should.equal('{"number":"1"}');
+          done();
+        });
+    });
+
+    after(function () {
+      // eslint-disable-next-line node/no-unsupported-features/es-builtins
+      delete BigInt.prototype.toJSON;
+    });
+  });
+
+
   it('should error for BigInt primitive', (done) => {
     try {
       request