diff --git a/segmetrics/measure.py b/segmetrics/measure.py
index 87a706d..6f1b13f 100644
--- a/segmetrics/measure.py
+++ b/segmetrics/measure.py
@@ -43,7 +43,7 @@ def compute(self, actual):
             An image containing uniquely labeled object masks corresponding to
             the segmentation results.
         """
-        return NotImplemented
+        return NotImplementedError()
 
     def default_name(self):
         """