diff --git a/src/dehb/optimizers/de.py b/src/dehb/optimizers/de.py
index 794dce6..4c27166 100644
--- a/src/dehb/optimizers/de.py
+++ b/src/dehb/optimizers/de.py
@@ -251,7 +251,7 @@ def __getstate__(self):
     def __del__(self):
         """ Ensures a clean kill of the Dask client and frees up a port.
         """
-        if hasattr(self, "client") and isinstance(self, Client):
+        if hasattr(self, "client") and isinstance(self.client, Client):
             self.client.close()
 
     def reset(self):
diff --git a/src/dehb/optimizers/dehb.py b/src/dehb/optimizers/dehb.py
index 5ec8985..79bd584 100644
--- a/src/dehb/optimizers/dehb.py
+++ b/src/dehb/optimizers/dehb.py
@@ -35,7 +35,7 @@ def __init__(self, cs=None, f=None, dimensions=None, mutation_factor=None,
         self.configspace = True if isinstance(self.cs, ConfigSpace.ConfigurationSpace) else False
         if self.configspace:
             self.dimensions = len(self.cs.get_hyperparameters())
-        elif dimensions is None or not isinstance(dimensions, (int, np.int)):
+        elif dimensions is None or not isinstance(dimensions, (int, np.integer)):
             assert "Need to specify `dimensions` as an int when `cs` is not available/specified!"
         else:
             self.dimensions = dimensions