diff --git a/src/revpimodio2/_internal.py b/src/revpimodio2/_internal.py
index 80cc8bb..b1415ab 100644
--- a/src/revpimodio2/_internal.py
+++ b/src/revpimodio2/_internal.py
@@ -68,5 +68,7 @@ def consttostr(value) -> str:
         return "OUT"
     elif value == 302:
         return "MEM"
+    elif value == 4096:
+        return "PROCESS_IMAGE_SIZE"
     else:
         return ""
diff --git a/tests/test_import.py b/tests/test_import.py
index eb19538..eb41847 100644
--- a/tests/test_import.py
+++ b/tests/test_import.py
@@ -12,3 +12,27 @@ class ModuleImport(unittest.TestCase):
         import revpimodio2
 
         self.assertEqual(type(revpimodio2.__version__), str)
+
+    def test_lib_constants(self):
+        """Tests constants of _internal module."""
+        import revpimodio2
+
+        self.assertEqual(revpimodio2._internal.consttostr(999), "")
+
+        lst_const = [0, 1, 2, 4, 31, 32, 33, 300, 301, 302, 4096]
+        internal_dict = revpimodio2._internal.__dict__  # type: dict
+        for key in internal_dict:
+            if type(internal_dict[key]) is int:
+                const_value = internal_dict[key]
+                self.assertEqual(revpimodio2._internal.consttostr(const_value), key)
+                self.assertTrue(const_value in lst_const)
+
+        # Test argument checker
+        revpimodio2._internal.acheck(bool, arg01=True, arg02_noneok=None)
+        revpimodio2._internal.acheck(int, arg01=0, arg02_noneok=10)
+        revpimodio2._internal.acheck(str, arg01="", arg02_noneok="ja")
+
+        with self.assertRaises(TypeError):
+            revpimodio2._internal.acheck(str, arg01=None, arg02_noneok="test")
+        with self.assertRaises(TypeError):
+            revpimodio2._internal.acheck(bool, arg01=True, arg02=None)