diff --git a/audio_splitter/core.py b/audio_splitter/core.py
index 3bc3f34..5f56d0b 100644
--- a/audio_splitter/core.py
+++ b/audio_splitter/core.py
@@ -7,6 +7,7 @@ import sys
 from typing import List, Dict, Any
 
 from .constants import (
+    CONTAINER_INFO,
     CONTAINER_NAMES,
     CODEC_TO_EXTENSION_MAP,
     CODEC_NAME_TO_FFMPEG,
@@ -117,12 +118,15 @@ def split_audio(input_file: str, output_directory: str, tracks: List[Dict[str, A
     else:
         output_audio_codec = stream_info.get('audio_codec', '')
 
-    # Choose extension based on audio codec if possible, otherwise fallback to container default
-    if output_audio_codec in CODEC_TO_EXTENSION_MAP:
+    # Choose extension based on the output container, not the audio codec.
+    # When user specifies --container, the file extension must match the container.
+    container_info = next((c for c in CONTAINER_INFO if c['name'] == output_container), None)
+    if container_info:
+        extension = container_info['extension']
+    elif output_audio_codec in CODEC_TO_EXTENSION_MAP:
         extension = CODEC_TO_EXTENSION_MAP[output_audio_codec]
     else:
-        container_info = next((c for c in CONTAINER_INFO if c['name'] == output_container), None)
-        extension = container_info['extension'] if container_info else '.mkv'
+        extension = '.mkv'
 
     print(f"Output extension: {extension}")