diff --git a/pypop7/optimizers/core/optimizer.py b/pypop7/optimizers/core/optimizer.py
index 27d013f0b..fabe97aa6 100644
--- a/pypop7/optimizers/core/optimizer.py
+++ b/pypop7/optimizers/core/optimizer.py
@@ -60,6 +60,7 @@ def __init__(self, problem, options):
         self.options = options  # not use `deep copy` to save memory
         # for `MAX_FUNCTION_EVALUATIONS` in `Terminations`
         self.max_function_evaluations = options.get('max_function_evaluations', np.inf)
+        # for `MAX_RUNTIME` in `MAX_RUNTIME`
         self.max_runtime = options.get('max_runtime', np.inf)
         self.fitness_threshold = options.get('fitness_threshold', -np.inf)
         self.n_individuals = options.get('n_individuals')  # offspring population size