diff --git a/elephant/signal_processing.py b/elephant/signal_processing.py
index 9a09f3fa8..1e8fe8631 100644
--- a/elephant/signal_processing.py
+++ b/elephant/signal_processing.py
@@ -167,7 +167,7 @@ def zscore(signal, inplace=True):
     for sig in signal:
         # Perform inplace operation only if array is of dtype float.
         # Otherwise, raise an error.
-        if inplace and not np.issubdtype(float, sig.dtype):
+        if inplace and not np.issubdtype(sig.dtype, np.floating):
             raise ValueError(f"Cannot perform inplace operation as the "
                              f"signal dtype is not float. Source: {sig.name}")
 
diff --git a/elephant/test/test_signal_processing.py b/elephant/test/test_signal_processing.py
index 303e3cdfd..83e51d4f6 100644
--- a/elephant/test/test_signal_processing.py
+++ b/elephant/test/test_signal_processing.py
@@ -401,7 +401,7 @@ def test_zscore_list_inplace(self):
         self.assertIs(result[0], signal_list[0])
         self.assertIs(result[1], signal_list[1])
 
-    def test_wrong_input(self):
+    def test_z_score_wrong_input(self):
         # wrong type
         self.assertRaises(TypeError, elephant.signal_processing.zscore,
                           signal=[1, 2] * pq.uV)
@@ -411,6 +411,23 @@ def test_wrong_input(self):
         self.assertRaises(ValueError, elephant.signal_processing.zscore,
                           signal=[asig1, asig2])
 
+    def test_z_score_np_float32_64(self):
+        """
+        Regression test: Inplace operations for z_score failed when using
+        np.float32 or np.float64 types.
+        See Issue #591.
+        https://github.com/NeuralEnsemble/elephant/issues/591
+        """
+        test_types = (np.float32, np.float64)
+        for test_type in test_types:
+            with self.subTest(test_type):
+                signal = neo.AnalogSignal(self.test_seq1, units='mV',
+                                          t_start=0. * pq.ms,
+                                          sampling_rate=1000. * pq.Hz,
+                                          dtype=test_type)
+                # This should not raise a ValueError
+                elephant.signal_processing.zscore(signal, inplace=True)
+
 
 class ButterTestCase(unittest.TestCase):