diff --git a/src/boxes/defaults.ts b/src/boxes/defaults.ts
index 1bdf5bc1..d50707e7 100644
--- a/src/boxes/defaults.ts
+++ b/src/boxes/defaults.ts
@@ -233,7 +233,7 @@ export class mvexBox extends ContainerBox {
   static override readonly fourcc = 'mvex' as const;
   box_name = 'MovieExtendsBox' as const;
   trex: trexBox;
-  mehd: mehdBox;
+  mehd?: mehdBox;
   mehds: Array<mehdBox>;
 
   trexs: Array<trexBox> = [];
diff --git a/src/boxes/stsz.ts b/src/boxes/stsz.ts
index 1573c9cf..ba8b45dd 100644
--- a/src/boxes/stsz.ts
+++ b/src/boxes/stsz.ts
@@ -30,17 +30,7 @@ export class stszBox extends FullBox {
     let constant = true;
     this.version = 0;
     this.flags = 0;
-    if (this.sample_sizes.length > 0) {
-      let i = 0;
-      while (i + 1 < this.sample_sizes.length) {
-        if (this.sample_sizes[i + 1] !== this.sample_sizes[0]) {
-          constant = false;
-          break;
-        } else {
-          i++;
-        }
-      }
-    } else {
+    if (this.sample_sizes.length > 0 && this.sample_size === 0) {
       constant = false;
     }
     this.size = 8;
@@ -48,11 +38,7 @@ export class stszBox extends FullBox {
       this.size += 4 * this.sample_sizes.length;
     }
     this.writeHeader(stream);
-    if (!constant) {
-      stream.writeUint32(0);
-    } else {
-      stream.writeUint32(this.sample_sizes[0]);
-    }
+    stream.writeUint32(this.sample_size);
     stream.writeUint32(this.sample_sizes.length);
     if (!constant) {
       stream.writeUint32Array(this.sample_sizes);
diff --git a/src/isofile.ts b/src/isofile.ts
index d2d8c334..f0a5f41e 100644
--- a/src/isofile.ts
+++ b/src/isofile.ts
@@ -1337,7 +1337,7 @@ export class ISOFile<TSegmentUser = unknown, TSampleUser = unknown> {
       buffer: ISOFile.writeInitializationSegment(
         this.ftyp,
         moov,
-        this.moov?.mvex?.mehd.fragment_duration,
+        this.moov?.mvex?.mehd?.fragment_duration,
       ),
     };
   }
diff --git a/src/parser.ts b/src/parser.ts
index 7a560b64..e9c5a8ce 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -87,6 +87,8 @@ export function parseOneBox(
         box = new Box(size);
         box.type = type as BoxFourCC;
         return { code: OK, box, size: box.size };
+      } else {
+        size = stream.getEndPosition() - start;
       }
     }
   }
diff --git a/tests/conformance.test.ts b/tests/conformance.test.ts
index 28ac5b49..36c3f52f 100644
--- a/tests/conformance.test.ts
+++ b/tests/conformance.test.ts
@@ -253,16 +253,7 @@ describe('Conformance Tests', () => {
       file_groups[groupName].filter(test => {
         // FIXME: These files might be correct but I haven't investigated them yet
         // Majority of the tests pass so I've left them out for now
-        const ignoredTests = [
-          'video_2500000bps_0.mp4',
-          'a3b-tone-deprot.mp4',
-          'a4-tone-fragmented.mp4',
-          'a6_tone_multifile.mp4',
-          'a7-tone-oddities.mp4',
-          'a9-aac-samplegroups-edit.mp4',
-          'hevc/hevc_tiles_multiple_tracks_empty_base.mp4',
-          'hevc/hevc_tiles_multiple_tracks.mp4',
-        ];
+        const ignoredTests = ['video_2500000bps_0.mp4'];
         return !ignoredTests.includes(test);
       }),
     )('roundtrip %s', async testName => {