diff --git a/gluon/portalocker.py b/gluon/portalocker.py
index 2bcd63f5..de584323 100644
--- a/gluon/portalocker.py
+++ b/gluon/portalocker.py
@@ -119,7 +119,7 @@ class LockedFile(object):
             lock(self.file, LOCK_EX)
             if not 'a' in mode:
                 self.file.seek(0)
-                self.file.truncate()
+                self.file.truncate(0)
         else:
             raise RuntimeError("invalid LockedFile(...,mode)")