diff --git a/pypop7/optimizers/cc/_test_hcc.py b/pypop7/optimizers/cc/_test_hcc.py
index 39e66c4bc..9cace526e 100644
--- a/pypop7/optimizers/cc/_test_hcc.py
+++ b/pypop7/optimizers/cc/_test_hcc.py
@@ -20,8 +20,8 @@ def test_optimize(self):
             print('*' * 7 + ' ' + f.__name__ + ' ' + '*' * 7)
             problem = {'fitness_function': f,
                        'ndim_problem': ndim_problem,
-                       'lower_boundary': -5*np.ones((ndim_problem,)),
-                       'upper_boundary': 5*np.ones((ndim_problem,))}
+                       'lower_boundary': -5.0*np.ones((ndim_problem,)),
+                       'upper_boundary': 5.0*np.ones((ndim_problem,))}
             options = {'max_function_evaluations': 2e6,
                        'fitness_threshold': 1e-10,
                        'seed_rng': 0,