diff --git a/apiclient/http.py b/apiclient/http.py
index 9fdd39c..21455f9 100644
--- a/apiclient/http.py
+++ b/apiclient/http.py
@@ -265,7 +265,7 @@
     Returns:
       Size of the body.
     """
-    return len(self.body)
+    return len(self._body)
 
   def resumable(self):
     """Whether this upload is resumable.