diff --git a/trunk/src/app/srs_app_rtc.cpp b/trunk/src/app/srs_app_rtc.cpp
index 8424216ce..64c654fc7 100644
--- a/trunk/src/app/srs_app_rtc.cpp
+++ b/trunk/src/app/srs_app_rtc.cpp
@@ -105,8 +105,10 @@ srs_error_t SrsRtpH264Muxer::frame_to_packet(SrsSharedPtrMessage* shared_frame,
     srs_error_t err = srs_success;
 
     if (format->is_avc_sequence_header()) {
+        // It is ok when size is 0, @see http://www.cplusplus.com/reference/string/string/assign/
         sps.assign(format->vcodec->sequenceParameterSetNALUnit.data(), format->vcodec->sequenceParameterSetNALUnit.size());
         pps.assign(format->vcodec->pictureParameterSetNALUnit.data(), format->vcodec->pictureParameterSetNALUnit.size());
+
         // only collect SPS/PPS.
         return err;
     }
diff --git a/trunk/src/main/srs_main_server.cpp b/trunk/src/main/srs_main_server.cpp
index e4641285c..8195d0020 100644
--- a/trunk/src/main/srs_main_server.cpp
+++ b/trunk/src/main/srs_main_server.cpp
@@ -209,7 +209,7 @@ srs_error_t do_main(int argc, char** argv)
 
 int main(int argc, char** argv) {
     srs_error_t err = do_main(argc, argv);
-    
+
     if (err != srs_success) {
         srs_error("Failed, %s", srs_error_desc(err).c_str());
     }
diff --git a/trunk/src/utest/srs_utest_kernel.cpp b/trunk/src/utest/srs_utest_kernel.cpp
index 29638dceb..72395fc61 100644
--- a/trunk/src/utest/srs_utest_kernel.cpp
+++ b/trunk/src/utest/srs_utest_kernel.cpp
@@ -4869,20 +4869,20 @@ VOID TEST(KernelMP4Test, CoverMP4CodecSingleFrame)
 
         // Sequence header.
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         // Frame group #0
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(87, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(87, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
     }
 
@@ -4984,20 +4984,20 @@ VOID TEST(KernelMP4Test, CoverMP4MultipleVideos)
 
         // Sequence header.
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         // Frames order by dts asc.
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(40, (int)dts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(40, (int)dts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
     }
 }
@@ -5082,24 +5082,24 @@ VOID TEST(KernelMP4Test, CoverMP4MultipleCTTs)
 
         // Sequence header.
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         // Frames order by dts asc.
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(40, (int)dts); EXPECT_EQ(80, (int)pts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(40, (int)dts); EXPECT_EQ(80, (int)pts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(80, (int)dts); EXPECT_EQ(40, (int)pts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(80, (int)dts); EXPECT_EQ(40, (int)pts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
     }
 }
@@ -5194,28 +5194,28 @@ VOID TEST(KernelMP4Test, CoverMP4MultipleAVs)
 
         // Sequence header.
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(41, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(2, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_EQ(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         // Frames order by dts asc.
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(0, (int)dts); EXPECT_EQ(87, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(0, (int)dts); EXPECT_EQ(87, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(20, (int)dts); EXPECT_EQ(87, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(20, (int)dts); EXPECT_EQ(87, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeSOUN, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
 
         HELPER_EXPECT_SUCCESS(dec.read_sample(&ht, &ft, &ct, &dts, &pts, &sample, &nb_sample));
-        EXPECT_EQ(40, (int)dts); EXPECT_EQ(40, (int)pts); EXPECT_EQ(127, nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
+        EXPECT_EQ(40, (int)dts); EXPECT_EQ(40, (int)pts); EXPECT_EQ(127, (int)nb_sample); EXPECT_EQ(SrsMp4HandlerTypeVIDE, ht); EXPECT_NE(SrsAudioAacFrameTraitSequenceHeader, ct);
         srs_freepa(sample);
     }
 }
@@ -5338,3 +5338,16 @@ VOID TEST(KernelMP4Test, CoverMP4M2tsSegmentEncoder)
     HELPER_EXPECT_SUCCESS(enc.flush(dts));
 }
 
+VOID TEST(KernelUtilityTest, CoverStringAssign)
+{
+    string sps = "SRS";
+    ASSERT_STREQ("SRS", sps.c_str());
+
+    sps.assign("Hello", 5);
+    ASSERT_STREQ("Hello", sps.c_str());
+
+    sps.assign("World", 0);
+    ASSERT_EQ(0, (int)sps.length());
+    ASSERT_STREQ("", sps.c_str());
+}
+