diff --git a/src/eko/constants.py b/src/eko/constants.py
index c01197405..fba786f46 100644
--- a/src/eko/constants.py
+++ b/src/eko/constants.py
@@ -84,6 +84,8 @@ def uplike_flavors(nf):
     nu : int
 
     """
+    if nf > 6:
+        raise NotImplementedError("Selected nf is not implemented")
     nu = nf // 2
     return nu