diff --git a/api/audio_codecs/audio_decoder_factory.h b/api/audio_codecs/audio_decoder_factory.h
index c36a0e103b..2811f6704b 100644
--- a/api/audio_codecs/audio_decoder_factory.h
+++ b/api/audio_codecs/audio_decoder_factory.h
@@ -38,6 +38,8 @@ class AudioDecoderFactory : public rtc::RefCountInterface {
   // communication between the AudioEncoder and AudioDecoder instances, which is
   // needed for some codecs with built-in bandwidth adaptation.)
   //
+  // Returns null if the format isn't supported.
+  //
   // Note: Implementations need to be robust against combinations other than
   // one encoder, one decoder getting the same ID; such decoders must still
   // work.
diff --git a/api/audio_codecs/audio_encoder_factory.h b/api/audio_codecs/audio_encoder_factory.h
index 48995a876d..6128b1b6f3 100644
--- a/api/audio_codecs/audio_encoder_factory.h
+++ b/api/audio_codecs/audio_encoder_factory.h
@@ -44,6 +44,8 @@ class AudioEncoderFactory : public rtc::RefCountInterface {
   // communication between the AudioEncoder and AudioDecoder instances, which is
   // needed for some codecs with built-in bandwidth adaptation.)
   //
+  // Returns null if the format isn't supported.
+  //
   // Note: Implementations need to be robust against combinations other than
   // one encoder, one decoder getting the same ID; such encoders must still
   // work.