diff --git a/content/media/moz.build b/content/media/moz.build
index c0685af7cc9e..bd2363c269a9 100644
--- a/content/media/moz.build
+++ b/content/media/moz.build
@@ -104,7 +104,7 @@ EXPORTS.mozilla.dom += [
     'VideoStreamTrack.h',
 ]
 
-SOURCES += [
+UNIFIED_SOURCES += [
     'AudioAvailableEventManager.cpp',
     'AudioChannelFormat.cpp',
     'AudioNodeEngine.cpp',
@@ -114,7 +114,6 @@ SOURCES += [
     'AudioStream.cpp',
     'AudioStreamTrack.cpp',
     'BufferDecoder.cpp',
-    'DecoderTraits.cpp',
     'DOMMediaStream.cpp',
     'EncodedBufferCache.cpp',
     'FileBlockCache.cpp',
@@ -144,6 +143,11 @@ SOURCES += [
     'WebVTTListener.cpp',
 ]
 
+# DecoderTraits.cpp needs to be built separately because of Mac OS X headers.
+SOURCES += [
+    'DecoderTraits.cpp',
+]
+
 FAIL_ON_WARNINGS = True
 
 if CONFIG['CPU_ARCH'] == 'arm' and CONFIG['BUILD_ARM_NEON']: