diff --git a/lib/hls/hls_parser.js b/lib/hls/hls_parser.js
index 3c03b6cedc..9e0df8da0b 100644
--- a/lib/hls/hls_parser.js
+++ b/lib/hls/hls_parser.js
@@ -1893,9 +1893,20 @@ shaka.hls.HlsParser = class {
         const pPreviousReference = i == 0 ?
           previousReference : partialSegmentRefs[partialSegmentRefs.length - 1];
         const pStartTime = (i == 0) ? startTime : pPreviousReference.endTime;
-        const pDuration = Number(item.getAttributeValue('DURATION'));
-        // A preload hinted partial segment doesn't have duration information,
-        // so its startTime and endTime are the same.
+
+        // If DURATION is missing from this partial segment, use the target
+        // partial duration from the top of the playlist, which is a required
+        // attribute for content with partial segments.
+        const pDuration = Number(item.getAttributeValue('DURATION')) ||
+            this.partialTargetDuration_;
+
+        // If for some reason we have neither an explicit duration, nor a target
+        // partial duration, we should SKIP this partial segment to avoid
+        // duplicating content in the presentation timeline.
+        if (!pDuration) {
+          continue;
+        }
+
         const pEndTime = pStartTime + pDuration;
 
         let pStartByte = 0;
diff --git a/test/hls/hls_live_unit.js b/test/hls/hls_live_unit.js
index e5994a1d0f..655a0317c8 100644
--- a/test/hls/hls_live_unit.js
+++ b/test/hls/hls_live_unit.js
@@ -539,11 +539,13 @@ describe('HlsParser live', () => {
       ManifestParser.verifySegmentIndex(video, [ref1, ref2]);
     });
 
+    // Test for https://github.com/shaka-project/shaka-player/issues/4223
     it('parses streams with partial and preload hinted segments', async () => {
       playerInterface.isLowLatencyMode = () => true;
       const mediaWithPartialSegments = [
         '#EXTM3U\n',
         '#EXT-X-TARGETDURATION:5\n',
+        '#EXT-X-PART-INF:PART-TARGET=1.5\n',
         '#EXT-X-MAP:URI="init.mp4",BYTERANGE="616@0"\n',
         '#EXT-X-MEDIA-SEQUENCE:0\n',
         // ref includes partialRef, partialRef2
@@ -576,26 +578,73 @@ describe('HlsParser live', () => {
           'test:/partial2.mp4', 2, 4, /* syncTime= */ null,
           /* baseUri= */ '', /* startByte= */ 200, /* endByte= */ 429);
 
+      const ref = makeReference(
+          'test:/main.mp4', 0, 4, /* syncTime= */ null,
+          /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ 429,
+          /* timestampOffset= */ 0, [partialRef, partialRef2]);
+
       const partialRef3 = makeReference(
           'test:/partial.mp4', 4, 6, /* syncTime= */ null,
           /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ 209);
 
-      // A preload hinted partial segment doesn't have duration information,
-      // so its startTime and endTime are the same.
       const preloadRef = makeReference(
-          'test:/partial.mp4', 6, 6, /* syncTime= */ null,
+          'test:/partial.mp4', 6, 7.5, /* syncTime= */ null,
           /* baseUri= */ '', /* startByte= */ 210, /* endByte= */ null);
 
+      // ref2 is not fully published yet, so it doesn't have a segment uri.
+      const ref2 = makeReference(
+          '', 4, 7.5, /* syncTime= */ null,
+          /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ null,
+          /* timestampOffset= */ 0, [partialRef3, preloadRef]);
+
+      const manifest = await parser.start('test:/master', playerInterface);
+      const video = manifest.variants[0].video;
+      await video.createSegmentIndex();
+      ManifestParser.verifySegmentIndex(video, [ref, ref2]);
+    });
+
+    // Test for https://github.com/shaka-project/shaka-player/issues/4223
+    it('ignores preload hinted segments without target duration', async () => {
+      playerInterface.isLowLatencyMode = () => true;
+
+      // Missing PART-TARGET, so preload hints are skipped.
+      const mediaWithPartialSegments = [
+        '#EXTM3U\n',
+        '#EXT-X-TARGETDURATION:5\n',
+        '#EXT-X-MAP:URI="init.mp4",BYTERANGE="616@0"\n',
+        '#EXT-X-MEDIA-SEQUENCE:0\n',
+        '#EXTINF:4,\n',
+        // ref1
+        'main.mp4\n',
+        // ref2 includes partialRef, but not preloadRef
+        // partialRef
+        '#EXT-X-PART:DURATION=2,URI="partial.mp4",BYTERANGE=210@0\n',
+        // preloadRef
+        '#EXT-X-PRELOAD-HINT:TYPE=PART,URI="partial.mp4",BYTERANGE-START=210\n',
+      ].join('');
+
+      fakeNetEngine
+          .setResponseText('test:/master', master)
+          .setResponseText('test:/video', mediaWithPartialSegments)
+          .setResponseValue('test:/init.mp4', initSegmentData)
+          .setResponseValue('test:/main.mp4', segmentData)
+          .setResponseValue('test:/partial.mp4', segmentData)
+          .setResponseValue('test:/partial2.mp4', segmentData);
+
       const ref = makeReference(
           'test:/main.mp4', 0, 4, /* syncTime= */ null,
-          /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ 429,
-          /* timestampOffset= */ 0, [partialRef, partialRef2]);
+          /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ null,
+          /* timestampOffset= */ 0, []);
+
+      const partialRef = makeReference(
+          'test:/partial.mp4', 4, 6, /* syncTime= */ null,
+          /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ 209);
 
       // ref2 is not fully published yet, so it doesn't have a segment uri.
       const ref2 = makeReference(
           '', 4, 6, /* syncTime= */ null,
-          /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ null,
-          /* timestampOffset= */ 0, [partialRef3, preloadRef]);
+          /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ 209,
+          /* timestampOffset= */ 0, [partialRef]);
 
       const manifest = await parser.start('test:/master', playerInterface);
       const video = manifest.variants[0].video;
@@ -611,6 +660,7 @@ describe('HlsParser live', () => {
       const mediaWithPartialSegments = [
         '#EXTM3U\n',
         '#EXT-X-TARGETDURATION:5\n',
+        '#EXT-X-PART-INF:PART-TARGET=1.5\n',
         '#EXTINF:4,\n',
         'main.mp4\n',
         '#EXT-X-PART:DURATION=2,URI="partial.mp4",BYTERANGE=210@0\n',