diff --git a/tests/core/test_ion.py b/tests/core/test_ion.py
index d1b40e6e5c2..407fe1c67a3 100644
--- a/tests/core/test_ion.py
+++ b/tests/core/test_ion.py
@@ -4,6 +4,7 @@
 from unittest import TestCase
 
 import pytest
+
 from pymatgen.core import Composition, Element
 from pymatgen.core.ion import Ion
 
@@ -59,6 +60,7 @@ def test_special_formulas(self):
             ("O3", "O3(aq)"),
             ("O2", "O2(aq)"),
             ("N2", "N2(aq)"),
+            ("NaOH", "NaOH(aq)"),
             ("H4O4", "H2O2(aq)"),
             ("OH-", "OH[-1]"),
             ("H2PO4-", "H2PO4[-1]"),