diff --git a/darshan-util/pydarshan/darshan/experimental/aggregators/mod_agg_iohist.py b/darshan-util/pydarshan/darshan/experimental/aggregators/mod_agg_iohist.py
index 232b95428..67589081a 100644
--- a/darshan-util/pydarshan/darshan/experimental/aggregators/mod_agg_iohist.py
+++ b/darshan-util/pydarshan/darshan/experimental/aggregators/mod_agg_iohist.py
@@ -58,7 +58,7 @@ def fix_name(name):
 
 
     if mode == 'append':
-        if 'agg_iohist' not in self.data:
+        if 'agg_iohist' not in self.summary:
             self.summary['agg_iohist'] = {}
         self.summary['agg_iohist'][mod] = ctx