diff --git a/spec/timezone.html b/spec/timezone.html
index a2daab54c..ea5f5204b 100644
--- a/spec/timezone.html
+++ b/spec/timezone.html
@@ -309,8 +309,12 @@ <h1>
           1. Throw a *RangeError* exception.
         1. Let _before_ be the latest possible ISO Date-Time Record for which CompareISODateTime(_before_, _isoDateTime_) = -1 and ! GetPossibleEpochNanoseconds(_timeZone_, _before_) is not empty.
         1. Let _after_ be the earliest possible ISO Date-Time Record for which CompareISODateTime(_after_, _isoDateTime_) = 1 and ! GetPossibleEpochNanoseconds(_timeZone_, _after_) is not empty.
-        1. Let _offsetBefore_ be GetOffsetNanosecondsFor(_timeZone_, _before_).
-        1. Let _offsetAfter_ be GetOffsetNanosecondsFor(_timeZone_, _after_).
+        1. Let _beforePossible_ be ! GetPossibleEpochNanoseconds(_timeZone_, _before_).
+        1. Assert: _beforePossible_'s length is 1.
+        1. Let _afterPossible_ be ! GetPossibleEpochNanoseconds(_timeZone_, _after_).
+        1. Assert: _afterPossible_'s length is 1.
+        1. Let _offsetBefore_ be GetOffsetNanosecondsFor(_timeZone_, _beforePossible_[0]).
+        1. Let _offsetAfter_ be GetOffsetNanosecondsFor(_timeZone_, _afterPossible_[0]).
         1. Let _nanoseconds_ be _offsetAfter_ - _offsetBefore_.
         1. Assert: abs(_nanoseconds_) ≤ nsPerDay.
         1. If _disambiguation_ is ~earlier~, then