diff --git a/src/mapper/dependency.py b/src/mapper/dependency.py
index 396be4f38c3e2fa84837a54f611bcf6e476647eb..d4e7b87cf14d7d985fc4c236fb6a9755b60a48bf 100644
--- a/src/mapper/dependency.py
+++ b/src/mapper/dependency.py
@@ -20,6 +20,8 @@ class DependencyMapper(CombineMapper):
             include_lookups = True
             include_calls = True
 
+        assert include_calls in [True, False, "descend_args"]
+
         self.include_subscripts = include_subscripts
         self.include_lookups = include_lookups
         self.include_calls = include_calls