--- a/toolkit/components/osfile/osfile_unix_front.jsm
+++ b/toolkit/components/osfile/osfile_unix_front.jsm
@@ -137,17 +137,17 @@
       * file.  If |OS.File.POS_CURRENT|, |pos| is relative to the
       * current position in the file. If |OS.File.POS_END|, |pos| is
       * relative to the end of the file.
       *
       * @return The new position in the file.
       */
      File.prototype.setPosition = function setPosition(pos, whence) {
        if (whence === undefined) {
-         whence = Const.SEEK_START;
+         whence = Const.SEEK_SET;
        }
        return throw_on_negative("setPosition",
          UnixFile.lseek(this.fd, pos, whence)
        );
      };
 
      /**
       * Fetch the information on the file.