diff --git a/spec.emu b/spec.emu
index 86c45b8..7352234 100644
--- a/spec.emu
+++ b/spec.emu
@@ -48,7 +48,9 @@ copyright: false
       1. Let _buffer_ be a new empty List.
       1. Repeat,
         1. Let _value_ be ? IteratorStepValue(_iterated_).
-        1. If _value_ is ~done~, return ReturnCompletion(*undefined*).
+        1. If _value_ is ~done~, then
+          1. If _buffer_ is not empty and the number of elements in _buffer_ < ℝ(_windowSize_), throw a *TypeError* exception.
+          1. Return ReturnCompletion(*undefined*).
         1. Append _value_ to _buffer_.
         1. If the number of elements in _buffer_ is ℝ(_windowSize_), then
           1. Let _completion_ be Completion(Yield(CreateArrayFromList(_buffer_))).