diff --git a/media/libstagefright/MPEG4Extractor.cpp b/media/libstagefright/MPEG4Extractor.cpp
index b8988e6..1ba1c6e 100644
--- a/media/libstagefright/MPEG4Extractor.cpp
+++ b/media/libstagefright/MPEG4Extractor.cpp
@@ -1368,9 +1368,6 @@
                 return err;
             }
 
-            const char *mime;
-            CHECK(mLastTrack->meta->findCString(kKeyMIMEType, &mime));
-
             if (max_size != 0) {
                 // Assume that a given buffer only contains at most 10 chunks,
                 // each chunk originally prefixed with a 2 byte length will
@@ -1387,6 +1384,8 @@
                     height = 1080;
                 }
 
+                const char *mime;
+                CHECK(mLastTrack->meta->findCString(kKeyMIMEType, &mime));
                 if (!strcmp(mime, MEDIA_MIMETYPE_VIDEO_AVC)) {
                     // AVC requires compression ratio of at least 2, and uses
                     // macroblocks
@@ -1400,6 +1399,10 @@
             }
             *offset += chunk_size;
 
+            // NOTE: setting another piece of metadata invalidates any pointers (such as the
+            // mimetype) previously obtained, so don't cache them.
+            const char *mime;
+            CHECK(mLastTrack->meta->findCString(kKeyMIMEType, &mime));
             // Calculate average frame rate.
             if (!strncasecmp("video/", mime, 6)) {
                 size_t nSamples = mLastTrack->sampleTable->countSamples();
diff --git a/media/libstagefright/MetaData.cpp b/media/libstagefright/MetaData.cpp
index ae6ae2d..7b60afc 100644
--- a/media/libstagefright/MetaData.cpp
+++ b/media/libstagefright/MetaData.cpp
@@ -89,6 +89,9 @@
     return setData(key, TYPE_RECT, &r, sizeof(r));
 }
 
+/**
+ * Note that the returned pointer becomes invalid when additional metadata is set.
+ */
 bool MetaData::findCString(uint32_t key, const char **value) {
     uint32_t type;
     const void *data;