diff --git a/mlair/run_modules/post_processing.py b/mlair/run_modules/post_processing.py
index 4387f359d05b7c2d06c98bb1ad346d058afc3056..265372cf7855903b18bc081cb255b0fc246a8ceb 100644
--- a/mlair/run_modules/post_processing.py
+++ b/mlair/run_modules/post_processing.py
@@ -155,7 +155,8 @@ class PostProcessing(RunEnvironment):
         :param _iter: internal counter to reduce unnecessary recursive calls (maximum number is 2, otherwise something
             went wrong).
         """
-        self.bootstrap_skill_scores = {}
+        if _iter == 0:
+            self.bootstrap_skill_scores = {}
         for boot_type in to_list(bootstrap_type):
             self.bootstrap_skill_scores[boot_type] = {}
             for boot_method in to_list(bootstrap_method):