diff --git a/tenacity/__init__.py b/tenacity/__init__.py
index 72eba04..ea72829 100644
--- a/tenacity/__init__.py
+++ b/tenacity/__init__.py
@@ -425,8 +425,10 @@ def next_action(rs: "RetryCallState") -> None:
             sleep = rs.upcoming_sleep
             rs.next_action = RetryAction(sleep)
             rs.idle_for += sleep
-            self.statistics["idle_for"] += sleep
-            self.statistics["attempt_number"] += 1
+            self.statistics["idle_for"] = self.statistics.get("idle_for", 0) + sleep
+            self.statistics["attempt_number"] = (
+                self.statistics.get("attempt_number", 1) + 1
+            )
 
         self._add_action_func(next_action)