diff --git a/media/libstagefright/MPEG4Extractor.cpp b/media/libstagefright/MPEG4Extractor.cpp
index e7fe0d1..76db3fb 100644
--- a/media/libstagefright/MPEG4Extractor.cpp
+++ b/media/libstagefright/MPEG4Extractor.cpp
@@ -1778,6 +1778,9 @@
         case FOURCC('b', 't', 'r', 't'):
         {
             *offset += chunk_size;
+            if (mLastTrack == NULL) {
+                return ERROR_MALFORMED;
+            }
 
             uint8_t buffer[12];
             if (chunk_data_size != sizeof(buffer)) {