diff --git a/wfdb/io/annotation.py b/wfdb/io/annotation.py
index 5cde1edf..b398fa07 100644
--- a/wfdb/io/annotation.py
+++ b/wfdb/io/annotation.py
@@ -1352,7 +1352,7 @@ def get_contained_labels(self, inplace=True):
         else:
             raise Exception("No annotation labels contained in object")
 
-        contained_labels = label_map.loc[index_vals, :]
+        contained_labels = label_map.loc[list(index_vals), :]
 
         # Add the counts
         for i in range(len(counts[0])):