diff --git a/trunk/src/app/srs_app_rtc_source.cpp b/trunk/src/app/srs_app_rtc_source.cpp
index c842caefdc..eaa2d32d86 100644
--- a/trunk/src/app/srs_app_rtc_source.cpp
+++ b/trunk/src/app/srs_app_rtc_source.cpp
@@ -908,18 +908,18 @@ srs_error_t SrsRtcRtpBuilder::on_audio(SrsSharedPtrMessage* msg)
         return srs_error_wrap(err, "format consume audio");
     }
 
-    // Try to init codec when startup or codec changed.
-    if (format->acodec && (err = init_codec(format->acodec->id)) != srs_success) {
-        return srs_error_wrap(err, "init codec");
-    }
-
     // Ignore if no format->acodec, it means the codec is not parsed, or unknown codec.
     // @issue https://github.com/ossrs/srs/issues/1506#issuecomment-562079474
     if (!format->acodec) {
         return err;
     }
 
-    // ts support audio codec: aac/mp3
+    // Try to init codec when startup or codec changed.
+    if ((err = init_codec(format->acodec->id)) != srs_success) {
+        return srs_error_wrap(err, "init codec");
+    }
+
+    // support audio codec: aac/mp3
     SrsAudioCodecId acodec = format->acodec->id;
     if (acodec != SrsAudioCodecIdAAC && acodec != SrsAudioCodecIdMP3) {
         return err;