diff --git a/src/amuse/ext/masc/cluster.py b/src/amuse/ext/masc/cluster.py
index 1946323..ae7cc1d 100644
--- a/src/amuse/ext/masc/cluster.py
+++ b/src/amuse/ext/masc/cluster.py
@@ -73,6 +73,8 @@ def new_fixed_mass_distribution(
                 mass_max=stellar_mass/number_of_stars,
             )
         initial_mass_function = new_fixed_mass_distribution
+    else:
+        raise ValueError("This imf is not implemented")
 
     if stellar_mass:
         # Add stars to cluster, until mass limit reached (inclusive!)