diff --git a/quacc/data.py b/quacc/data.py
index 7606605..2e58936 100644
--- a/quacc/data.py
+++ b/quacc/data.py
@@ -45,9 +45,9 @@ class ExtendedData:
         pred_proba: np.ndarray,
         ext: np.ndarray = None,
     ) -> np.ndarray | sp.csr_matrix:
-        to_append = pred_proba
-        if ext is not None:
-            to_append = np.concatenate([ext, pred_proba], axis=1)
+        to_append = ext
+        if ext is None:
+            to_append = pred_proba
 
         if isinstance(instances, sp.csr_matrix):
             _to_append = sp.csr_matrix(to_append)