diff --git a/src/stcal/outlier_detection/median.py b/src/stcal/outlier_detection/median.py
index b4fe5fec..2ff899fe 100644
--- a/src/stcal/outlier_detection/median.py
+++ b/src/stcal/outlier_detection/median.py
@@ -115,7 +115,7 @@ 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. Mus be between 0 and full_shape[0].
+            The index at which to append the data. Must be between 0 and full_shape[0].
             Required if using in-memory median computation.
         """
         if self.in_memory: