diff --git a/media/libdatasource/FileSource.cpp b/media/libdatasource/FileSource.cpp
index bbf7dda..3d34d0c 100644
--- a/media/libdatasource/FileSource.cpp
+++ b/media/libdatasource/FileSource.cpp
@@ -107,6 +107,9 @@
 
     Mutex::Autolock autoLock(mLock);
     if (mLength >= 0) {
+        if (offset < 0) {
+            return UNKNOWN_ERROR;
+        }
         if (offset >= mLength) {
             return 0;  // read beyond EOF.
         }