diff --git a/trunk/src/app/srs_app_rtc_source.cpp b/trunk/src/app/srs_app_rtc_source.cpp
index d429de9bb..497b263be 100644
--- a/trunk/src/app/srs_app_rtc_source.cpp
+++ b/trunk/src/app/srs_app_rtc_source.cpp
@@ -1192,7 +1192,7 @@ srs_error_t SrsRtcFromRtmpBridger::package_fu_a(SrsSharedPtrMessage* msg, SrsSam
     uint8_t header = sample->bytes[0];
     uint8_t nal_type = header & kNalTypeMask;
 
-    int num_of_packet = 1 + (sample->size - 1) / fu_payload_size;
+    int num_of_packet = 1 + (nb_left - 1) / fu_payload_size;
     for (int i = 0; i < num_of_packet; ++i) {
         int packet_size = srs_min(nb_left, fu_payload_size);