diff --git a/changelog/3632.feature.rst b/changelog/3632.feature.rst
index a715288e1..023fa3607 100644
--- a/changelog/3632.feature.rst
+++ b/changelog/3632.feature.rst
@@ -1 +1 @@
-Richer comparison introspection on ``AssertionError`` for objects created using `attrs <http://www.attrs.org/en/stable/>`_ or `dataclasses <https://docs.python.org/3/library/dataclasses.html>`_ (Python 3.7+).
+Richer equality comparison introspection on ``AssertionError`` for objects created using `attrs <http://www.attrs.org/en/stable/>`_ or `dataclasses <https://docs.python.org/3/library/dataclasses.html>`_ (Python 3.7+, `backported to Python 3.6 <https://pypi.org/project/dataclasses>`_).
diff --git a/src/_pytest/assertion/util.py b/src/_pytest/assertion/util.py
index ac83f6000..4536cd0dd 100644
--- a/src/_pytest/assertion/util.py
+++ b/src/_pytest/assertion/util.py
@@ -333,7 +333,7 @@ def _compare_eq_class(left, right, verbose, type=None):
         all_fields = left.__attrs_attrs__
         fields_to_check = [field.name for field in all_fields if field.cmp]
     else:
-        raise RuntimeError
+        raise RuntimeError("Unexpected value for `type` paramater")
 
     same = []
     diff = []