diff --git a/src/stcal/outlier_detection/median.py b/src/stcal/outlier_detection/median.py
index 2a408da1..b4fe5fec 100644
--- a/src/stcal/outlier_detection/median.py
+++ b/src/stcal/outlier_detection/median.py
@@ -115,8 +115,8 @@ def append(self,
             The data to append to the median computer. Must have shape full_shape[1:].
 
         idx
-            The index at which to append the data. Required if using in-memory
-            median computation.
+            The index at which to append the data. Mus be between 0 and full_shape[0].
+            Required if using in-memory median computation.
         """
         if self.in_memory:
             if idx is None: