diff --git a/benedict/dicts/__init__.py b/benedict/dicts/__init__.py
index 8b85ea9..cf5c05a 100644
--- a/benedict/dicts/__init__.py
+++ b/benedict/dicts/__init__.py
@@ -130,6 +130,11 @@ class benedict(KeypathDict, IODict, ParseDict):
         Return a new flattened dict using the given separator
         to join nested dict keys to flatten keypaths.
         """
+        if separator == self._keypath_separator:
+            raise ValueError(
+                f"Invalid flatten separator: '{separator}', "
+                "flatten separator must be different from keypath separator."
+            )
         return _flatten(self, separator)
 
     def get(self, key, default=None):
diff --git a/tests/github/test_issue_0088.py b/tests/github/test_issue_0088.py
new file mode 100644
index 0000000..c86f4b9
--- /dev/null
+++ b/tests/github/test_issue_0088.py
@@ -0,0 +1,36 @@
+# -*- coding: utf-8 -*-
+
+from benedict import benedict
+
+import unittest
+
+
+class github_issue_0088_test_case(unittest.TestCase):
+    """
+    This class describes a github issue 0088 test case.
+    https://github.com/fabiocaccamo/python-benedict/issues/88
+
+    To run this specific test:
+    - Run python -m unittest tests.github.test_issue_0088
+    """
+
+    def test_flatten_without_keypath_separator(self):
+        d = benedict({"a": {"b": {"c": 1}}}, keypath_separator=None)
+        f = d.flatten(".")
+        self.assertEqual(f, {"a.b.c": 1})
+
+    def test_flatten_with_separator_equal_to_keypath_separator(self):
+        d = benedict({"a": {"b": {"c": 1}}}, keypath_separator=".")
+        with self.assertRaises(ValueError):
+            f = d.flatten(".")
+        d = benedict({"a": {"b": {"c": 1}}}, keypath_separator="_")
+        with self.assertRaises(ValueError):
+            f = d.flatten("_")
+
+    def test_flatten_with_separator_different_from_keypath_separator(self):
+        d = benedict({"a": {"b": {"c": 1}}}, keypath_separator="_")
+        f = d.flatten(".")
+        self.assertEqual(f, {"a.b.c": 1})
+        d = benedict({"a": {"b": {"c": 1}}}, keypath_separator=".")
+        f = d.flatten("_")
+        self.assertEqual(f, {"a_b_c": 1})