diff --git a/python/ngen_cal/src/ngen/cal/_optimizers/grey_wolf.py b/python/ngen_cal/src/ngen/cal/_optimizers/grey_wolf.py
index 952f09f9..357865c3 100644
--- a/python/ngen_cal/src/ngen/cal/_optimizers/grey_wolf.py
+++ b/python/ngen_cal/src/ngen/cal/_optimizers/grey_wolf.py
@@ -145,7 +145,7 @@ def optimize(
 
         Returns
         -------
-        tuple
+        Tuple[float, np.ndarray]
             the global best cost and the global best position.
         """
         if n_processes is None: