diff --git a/CHANGELOG.md b/CHANGELOG.md
index 605f08be..98035f1a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 -   Fix parsing multi-byte UTF-8 characters on certain platforms.
 -   Fix crashes when attempting to encode audio frames without the correct
     number of `substream_id`s.
+-   Prevent encoding Mix Presentation OBUs with an inconsistent number of
+    annotations.
 
 ## [1.0.0] - 2024-01-26
 
diff --git a/iamf/obu/mix_presentation.cc b/iamf/obu/mix_presentation.cc
index 0c9541f8..95481439 100644
--- a/iamf/obu/mix_presentation.cc
+++ b/iamf/obu/mix_presentation.cc
@@ -37,7 +37,6 @@ absl::Status ValidateUniqueAudioElementIds(
     const std::vector<MixPresentationSubMix>& sub_mixes) {
   std::vector<DecodedUleb128> collected_audio_element_ids;
 
-  absl::flat_hash_set<DecodedUleb128> seen_audio_element_ids;
   // Audio Element IDs must be unique across all sub-mixes.
   for (const auto& sub_mix : sub_mixes) {
     for (const auto& audio_element : sub_mix.audio_elements) {
@@ -63,9 +62,14 @@ absl::Status ValidateUniqueAnchorElements(
 }
 
 absl::Status ValidateAndWriteSubMixAudioElement(
-    const SubMixAudioElement& element, WriteBitBuffer& wb) {
+    DecodedUleb128 count_label, const SubMixAudioElement& element,
+    WriteBitBuffer& wb) {
   // Write the main portion of an `SubMixAudioElement`.
   RETURN_IF_NOT_OK(wb.WriteUleb128(element.audio_element_id));
+  RETURN_IF_NOT_OK(ValidateVectorSizeEqual(
+      absl::StrCat("localized_element_annotations with audio_element_id= ",
+                   element.audio_element_id),
+      element.localized_element_annotations.size(), count_label));
   for (const auto& localized_element_annotation :
        element.localized_element_annotations) {
     RETURN_IF_NOT_OK(wb.WriteString(localized_element_annotation));
@@ -157,7 +161,8 @@ absl::Status ValidateAndWriteLayout(const MixPresentationLayout& layout,
   return absl::OkStatus();
 }
 
-absl::Status ValidateAndWriteSubMix(const MixPresentationSubMix& sub_mix,
+absl::Status ValidateAndWriteSubMix(DecodedUleb128 count_label,
+                                    const MixPresentationSubMix& sub_mix,
                                     WriteBitBuffer& wb) {
   // IAMF requires there to be at least one audio element.
   RETURN_IF_NOT_OK(ValidateNotEqual(
@@ -171,8 +176,8 @@ absl::Status ValidateAndWriteSubMix(const MixPresentationSubMix& sub_mix,
                                            sub_mix.audio_elements.size(),
                                            sub_mix.num_audio_elements));
   for (const auto& sub_mix_audio_element : sub_mix.audio_elements) {
-    RETURN_IF_NOT_OK(
-        ValidateAndWriteSubMixAudioElement(sub_mix_audio_element, wb));
+    RETURN_IF_NOT_OK(ValidateAndWriteSubMixAudioElement(
+        count_label, sub_mix_audio_element, wb));
   }
 
   RETURN_IF_NOT_OK(sub_mix.output_mix_gain.ValidateAndWrite(wb));
@@ -407,23 +412,30 @@ absl::Status MixPresentationObu::GetNumChannelsFromLayout(
 
 absl::Status MixPresentationObu::ValidateAndWritePayload(
     WriteBitBuffer& wb) const {
+  const std::string with_mix_presentation_id =
+      absl::StrCat(" with mix_presentation_id= ", mix_presentation_id_);
+
   // Write the main portion of the OBU.
   RETURN_IF_NOT_OK(wb.WriteUleb128(mix_presentation_id_));
   RETURN_IF_NOT_OK(wb.WriteUleb128(count_label_));
 
   RETURN_IF_NOT_OK(ValidateVectorSizeEqual(
       "language_labels", annotations_language_.size(), count_label_));
-  RETURN_IF_NOT_OK(ValidateUnique(annotations_language_.begin(),
-                                  annotations_language_.end(),
-                                  "Language labels"));
+  RETURN_IF_NOT_OK(ValidateUnique(
+      annotations_language_.begin(), annotations_language_.end(),
+      absl::StrCat("Language labels", with_mix_presentation_id)));
 
+  RETURN_IF_NOT_OK(ValidateVectorSizeEqual(
+      absl::StrCat("annotations_language", with_mix_presentation_id),
+      annotations_language_.size(), count_label_));
   for (const auto& annotations_language : annotations_language_) {
     RETURN_IF_NOT_OK(wb.WriteString(annotations_language));
   }
 
-  RETURN_IF_NOT_OK(ValidateVectorSizeEqual("mix presentation annotations",
-                                           annotations_language_.size(),
-                                           count_label_));
+  RETURN_IF_NOT_OK(ValidateVectorSizeEqual(
+      absl::StrCat("localized_presentation_annotation",
+                   with_mix_presentation_id),
+      localized_presentation_annotations_.size(), count_label_));
   for (const auto& localized_presentation_annotation :
        localized_presentation_annotations_) {
     RETURN_IF_NOT_OK(wb.WriteString(localized_presentation_annotation));
@@ -437,7 +449,7 @@ absl::Status MixPresentationObu::ValidateAndWritePayload(
   RETURN_IF_NOT_OK(
       ValidateVectorSizeEqual("sub_mixes", sub_mixes_.size(), num_sub_mixes_));
   for (const auto& sub_mix : sub_mixes_) {
-    RETURN_IF_NOT_OK(ValidateAndWriteSubMix(sub_mix, wb));
+    RETURN_IF_NOT_OK(ValidateAndWriteSubMix(count_label_, sub_mix, wb));
   }
 
   return absl::OkStatus();
diff --git a/iamf/obu/tests/mix_presentation_test.cc b/iamf/obu/tests/mix_presentation_test.cc
index cb2fd2d7..367703dd 100644
--- a/iamf/obu/tests/mix_presentation_test.cc
+++ b/iamf/obu/tests/mix_presentation_test.cc
@@ -255,6 +255,41 @@ TEST_F(MixPresentationObuTest, ValidateAndWriteFailsWithInvalidNumSubMixes) {
   EXPECT_FALSE(obu_->ValidateAndWriteObu(unused_wb).ok());
 }
 
+TEST_F(MixPresentationObuTest,
+       ValidateAndWriteFailsWithInconsistentCountLabelAndAnnotationsLanguage) {
+  count_label_ = 1;
+  annotations_language_.clear();
+
+  InitExpectOk();
+  WriteBitBuffer unused_wb(0);
+
+  EXPECT_FALSE(obu_->ValidateAndWriteObu(unused_wb).ok());
+}
+
+TEST_F(
+    MixPresentationObuTest,
+    ValidateAndWriteFailsWithInconsistentCountLabelAndLocalizedPresentationAnnotations) {
+  count_label_ = 1;
+  localized_presentation_annotations_.clear();
+
+  InitExpectOk();
+  WriteBitBuffer unused_wb(0);
+
+  EXPECT_FALSE(obu_->ValidateAndWriteObu(unused_wb).ok());
+}
+
+TEST_F(
+    MixPresentationObuTest,
+    ValidateAndWriteFailsWithInconsistentCountLabelAndLocalizedElementAnnotations) {
+  count_label_ = 1;
+  sub_mixes_[0].audio_elements[0].localized_element_annotations.clear();
+
+  InitExpectOk();
+  WriteBitBuffer unused_wb(0);
+
+  EXPECT_FALSE(obu_->ValidateAndWriteObu(unused_wb).ok());
+}
+
 TEST_F(MixPresentationObuTest,
        ValidateAndWriteFailsWithInvalidNonUniqueAudioElementIds) {
   ASSERT_EQ(sub_mixes_.size(), 1);