diff --git a/harness/temporalHelpers.js b/harness/temporalHelpers.js
index 017e2d59b5..68f32d33ba 100644
--- a/harness/temporalHelpers.js
+++ b/harness/temporalHelpers.js
@@ -1734,7 +1734,7 @@ var TemporalHelpers = {
       }
 
       getPossibleInstantsFor(plainDateTime) {
-        this.getPossibleInstantsForCalledWith.push(plainDateTime.toString());
+        this.getPossibleInstantsForCalledWith.push(plainDateTime.toString({ calendarName: "never" }));
         const [instant] = super.getPossibleInstantsFor(plainDateTime);
         if (this._shiftNanoseconds > 0) {
           if (this._isBeforeShift(instant)) return [instant];