diff --git a/pyam/statistics.py b/pyam/statistics.py
index 44e94149c..ae01a0494 100644
--- a/pyam/statistics.py
+++ b/pyam/statistics.py
@@ -211,7 +211,7 @@ def add(self, data, header, row=None, subheader=None):  # noqa: C901
         if self.stats is None:
             self.stats = _stats
         else:
-            self.stats = _stats.combine_first(self.stats)
+            self.stats = _stats.combine_first(self.stats, sort=False)
 
     def reindex(self, copy=True):
         """Reindex the summary statistics dataframe"""