diff --git a/harness/temporalHelpers.js b/harness/temporalHelpers.js
index 947ba282ec..33a8a9c9d8 100644
--- a/harness/temporalHelpers.js
+++ b/harness/temporalHelpers.js
@@ -933,6 +933,32 @@ var TemporalHelpers = {
     return new CalendarCheckFieldsPrototypePollution();
   },
 
+  /*
+   * A custom calendar used in prototype pollution checks. Verifies that the
+   * mergeFields() method is always called with null-prototype fields objects.
+   */
+  calendarCheckMergeFieldsPrototypePollution() {
+    class CalendarCheckMergeFieldsPrototypePollution extends Temporal.Calendar {
+      constructor() {
+        super("iso8601");
+        this.mergeFieldsCallCount = 0;
+      }
+
+      toString() {
+        return "merge-fields-null-proto";
+      }
+
+      mergeFields(fields, additionalFields) {
+        this.mergeFieldsCallCount++;
+        assert.sameValue(Object.getPrototypeOf(fields), null, "mergeFields should be called with null-prototype fields object (first argument)");
+        assert.sameValue(Object.getPrototypeOf(additionalFields), null, "mergeFields should be called with null-prototype fields object (second argument)");
+        return super.mergeFields(fields, additionalFields);
+      }
+    }
+
+    return new CalendarCheckMergeFieldsPrototypePollution();
+  },
+
   /*
    * A custom calendar used in prototype pollution checks. Verifies that methods
    * are always called with a null-prototype options object.
diff --git a/test/built-ins/Temporal/PlainDate/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js b/test/built-ins/Temporal/PlainDate/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
new file mode 100644
index 0000000000..d422c91b8b
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDate/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
@@ -0,0 +1,15 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.with
+description: >
+    Calendar.mergeFields method is called with null-prototype fields objects
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckMergeFieldsPrototypePollution();
+const instance = new Temporal.PlainDate(2000, 5, 2, calendar);
+instance.with({ day: 24 });
+assert.sameValue(calendar.mergeFieldsCallCount, 1, "mergeFields should have been called on the calendar");
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js b/test/built-ins/Temporal/PlainDateTime/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
new file mode 100644
index 0000000000..bbfe4145be
--- /dev/null
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
@@ -0,0 +1,15 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.with
+description: >
+    Calendar.mergeFields method is called with null-prototype fields objects
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckMergeFieldsPrototypePollution();
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, calendar);
+instance.with({ day: 24 });
+assert.sameValue(calendar.mergeFieldsCallCount, 1, "mergeFields should have been called on the calendar");
diff --git a/test/built-ins/Temporal/PlainMonthDay/prototype/toPlainDate/calendar-mergefields-called-with-null-prototype-fields.js b/test/built-ins/Temporal/PlainMonthDay/prototype/toPlainDate/calendar-mergefields-called-with-null-prototype-fields.js
new file mode 100644
index 0000000000..6561cef7f3
--- /dev/null
+++ b/test/built-ins/Temporal/PlainMonthDay/prototype/toPlainDate/calendar-mergefields-called-with-null-prototype-fields.js
@@ -0,0 +1,15 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.toplaindate
+description: >
+    Calendar.mergeFields method is called with null-prototype fields objects
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckMergeFieldsPrototypePollution();
+const instance = new Temporal.PlainMonthDay(5, 2, calendar);
+instance.toPlainDate({ year: 2019 });
+assert.sameValue(calendar.mergeFieldsCallCount, 1, "mergeFields should have been called on the calendar");
diff --git a/test/built-ins/Temporal/PlainMonthDay/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js b/test/built-ins/Temporal/PlainMonthDay/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
new file mode 100644
index 0000000000..0f2985bab6
--- /dev/null
+++ b/test/built-ins/Temporal/PlainMonthDay/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
@@ -0,0 +1,15 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.with
+description: >
+    Calendar.mergeFields method is called with null-prototype fields objects
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckMergeFieldsPrototypePollution();
+const instance = new Temporal.PlainMonthDay(5, 2, calendar);
+instance.with({ day: 24 });
+assert.sameValue(calendar.mergeFieldsCallCount, 1, "mergeFields should have been called on the calendar");
diff --git a/test/built-ins/Temporal/PlainYearMonth/prototype/toPlainDate/calendar-mergefields-called-with-null-prototype-fields.js b/test/built-ins/Temporal/PlainYearMonth/prototype/toPlainDate/calendar-mergefields-called-with-null-prototype-fields.js
new file mode 100644
index 0000000000..2b4f8fd901
--- /dev/null
+++ b/test/built-ins/Temporal/PlainYearMonth/prototype/toPlainDate/calendar-mergefields-called-with-null-prototype-fields.js
@@ -0,0 +1,15 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.toplaindate
+description: >
+    Calendar.mergeFields method is called with null-prototype fields objects
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckMergeFieldsPrototypePollution();
+const instance = new Temporal.PlainYearMonth(2000, 5, calendar);
+instance.toPlainDate({ day: 24 });
+assert.sameValue(calendar.mergeFieldsCallCount, 1, "mergeFields should have been called on the calendar");
diff --git a/test/built-ins/Temporal/PlainYearMonth/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js b/test/built-ins/Temporal/PlainYearMonth/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
new file mode 100644
index 0000000000..96b5e99466
--- /dev/null
+++ b/test/built-ins/Temporal/PlainYearMonth/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
@@ -0,0 +1,15 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.with
+description: >
+    Calendar.mergeFields method is called with null-prototype fields objects
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckMergeFieldsPrototypePollution();
+const instance = new Temporal.PlainYearMonth(2000, 5, calendar);
+instance.with({ year: 2019 });
+assert.sameValue(calendar.mergeFieldsCallCount, 1, "mergeFields should have been called on the calendar");
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js b/test/built-ins/Temporal/ZonedDateTime/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
new file mode 100644
index 0000000000..88192d8a16
--- /dev/null
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/with/calendar-mergefields-called-with-null-prototype-fields.js
@@ -0,0 +1,15 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.with
+description: >
+    Calendar.mergeFields method is called with null-prototype fields objects
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckMergeFieldsPrototypePollution();
+const instance = new Temporal.ZonedDateTime(0n, "UTC", calendar);
+instance.with({ day: 24 });
+assert.sameValue(calendar.mergeFieldsCallCount, 1, "mergeFields should have been called on the calendar");