diff --git a/toolkit/components/telemetry/GCTelemetry.jsm b/toolkit/components/telemetry/GCTelemetry.jsm
index 4847c42fc839..6b3bc2cdd858 100644
--- a/toolkit/components/telemetry/GCTelemetry.jsm
+++ b/toolkit/components/telemetry/GCTelemetry.jsm
@@ -129,7 +129,7 @@ function limitProperties(name, obj, count, log) {
       // properties.
       if (name === "data" && (
           key === "max_pause" ||
-          key === "num_slices" ||
+          key === "slices" ||
           key === "slices_list" ||
           key === "status" ||
           key === "timestamp" ||
@@ -156,9 +156,6 @@ function limitProperties(name, obj, count, log) {
  * etc.
  */
 function limitSize(data, log) {
-  // Store the number of slices so we know if we lost any at the end.
-  data.num_slices = data.slices_list.length;
-
   data.slices_list.sort((a, b) => b.pause - a.pause);
 
   if (data.slices_list.length > MAX_SLICES) {
diff --git a/toolkit/components/telemetry/tests/unit/test_TelemetryGC.js b/toolkit/components/telemetry/tests/unit/test_TelemetryGC.js
index ca0f9d5f64f7..bad5845a69a2 100644
--- a/toolkit/components/telemetry/tests/unit/test_TelemetryGC.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryGC.js
@@ -30,7 +30,7 @@ function run_test() {
   GCTelemetry.observeRaw(make_gc());
   // Get it back.
   assert_num_entries(1, false);
-  Assert.equal(20, Object.keys(get_entry()).length);
+  Assert.equal(19, Object.keys(get_entry()).length);
   // "true" will cause the entry to be clared.
   assert_num_entries(1, true);
   // There are currently no entries.
@@ -49,18 +49,18 @@ function run_test() {
 
   // Exactly the limit of fields.
   let my_gc_24 = make_gc();
-  for (let i = 0; i < 4; i++) {
+  for (let i = 0; i < 5; i++) {
       my_gc_24["new_property_" + i] = "Data";
   }
   GCTelemetry.observeRaw(my_gc_24);
-  // Assert that it was recorded but has only 7 fields.
+  // Assert that it was recorded has all 24 fields.
   Assert.equal(24, Object.keys(get_entry()).length);
   assert_num_entries(1, true);
   assert_num_entries(0, false);
 
   // Exactly too many fields.
   let my_gc_25 = make_gc();
-  for (let i = 0; i < 5; i++) {
+  for (let i = 0; i < 6; i++) {
       my_gc_25["new_property_" + i] = "Data";
   }
   GCTelemetry.observeRaw(my_gc_25);