diff --git a/rplidar.py b/rplidar.py
index 89ebd5c..47c878c 100644
--- a/rplidar.py
+++ b/rplidar.py
@@ -399,7 +399,7 @@ def iter_measures(self, scan_type='normal', max_buf_meas=3000):
             if self.scanning[2] == 'express':
                 if self.express_trame == 32:
                     self.express_trame = 0
-                    if not self.express_data:
+                    if not self.express_data or type(self.express_data) == bool:
                         self.logger.debug('reading first time bytes')
                         self.express_data = ExpressPacket.from_string(
                                             self._read_response(dsize))