@@ -330,6 +330,11 @@ def CheckConvertTimestampMicrosecondPadding(self):
             datetime.datetime(2012, 4, 4, 15, 6, 0, 123456),
         ])
 
+    def CheckInvalidIsolationLevelType(self):
+        # isolation level is a string, not an integer
+        self.assertRaises(TypeError,
+                          sqlite.connect, ":memory:", isolation_level=123)
+
 
 def suite():
     regression_suite = unittest.makeSuite(RegressionTests, "Check")
 
@@ -29,6 +29,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20026: Fix the sqlite module to handle correctly invalid isolation
+  level (wrong type).
+
 - Issue #18829: csv.Dialect() now checks type for delimiter, escapechar and
   quotechar fields.  Original patch by Vajrasky Kok.
 
 
@@ -109,7 +109,10 @@ int pysqlite_connection_init(pysqlite_Connection* self, PyObject* args, PyObject
         Py_INCREF(isolation_level);
     }
     self->isolation_level = NULL;
-    pysqlite_connection_set_isolation_level(self, isolation_level);
+    if (pysqlite_connection_set_isolation_level(self, isolation_level) < 0) {
+        Py_DECREF(isolation_level);
+        return -1;
+    }
     Py_DECREF(isolation_level);
 
     self->statement_cache = (pysqlite_Cache*)PyObject_CallFunction((PyObject*)&pysqlite_CacheType, "Oi", self, cached_statements);