diff --git a/lib/dash/dash_parser.js b/lib/dash/dash_parser.js
index 9086d27d40..169e49200b 100644
--- a/lib/dash/dash_parser.js
+++ b/lib/dash/dash_parser.js
@@ -345,6 +345,19 @@ shaka.dash.DashParser = class {
     let presentationTimeline;
     if (this.manifest_) {
       presentationTimeline = this.manifest_.presentationTimeline;
+
+      // Before processing an update, evict from all segment indexes.  Some of
+      // them may not get updated otherwise if their corresponding Period
+      // element has been dropped from the manifest since the last update.
+      // Without this, playback will still work, but this is necessary to
+      // maintain conditions that we assert on for multi-Period content.
+      // This gives us confidence that our state is maintained correctly, and
+      // that the complex logic of multi-Period eviction and period-flattening
+      // is correct.  See also:
+      // https://github.com/google/shaka-player/issues/3169#issuecomment-823580634
+      for (const segmentIndex of Object.values(this.segmentIndexMap_)) {
+        segmentIndex.evict(presentationTimeline.getSegmentAvailabilityStart());
+      }
     } else {
       // DASH IOP v3.0 suggests using a default delay between minBufferTime
       // and timeShiftBufferDepth.  This is literally the range of all
diff --git a/lib/media/segment_index.js b/lib/media/segment_index.js
index c0d20c581b..95ffe13b57 100644
--- a/lib/media/segment_index.js
+++ b/lib/media/segment_index.js
@@ -595,7 +595,7 @@ shaka.media.MetaSegmentIndex = class extends shaka.media.SegmentIndex {
   appendSegmentIndex(segmentIndex) {
     goog.asserts.assert(
         this.indexes_.length == 0 || segmentIndex.numEvicted == 0,
-        'Cannot have evicted segments in non-first Periods');
+        'Should not append a new segment index with already-evicted segments');
     this.indexes_.push(segmentIndex);
   }
 
@@ -655,7 +655,7 @@ shaka.media.MetaSegmentIndex = class extends shaka.media.SegmentIndex {
     for (const index of this.indexes_) {
       goog.asserts.assert(
           !sawSegments || index.numEvicted == 0,
-          'Cannot have evicted segments in non-first Periods');
+          'Should not see evicted segments after available segments');
       const reference = index.get(position - numPassedInEarlierIndexes);
 
       if (reference) {