diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index 19dabdd..db1e642 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -862,7 +862,9 @@
                 not_found.append(kall)
         if not_found:
             raise AssertionError(
-                '%r not all found in call list' % (tuple(not_found),)
+                '%r does not contain all of %r in its call list, '
+                'found %r instead' % (self._mock_name or 'mock',
+                                      tuple(not_found), all_calls)
             ) from cause
 
 
diff --git a/Misc/NEWS.d/next/Tests/2018-07-10-18-53-46.bpo-0.UBQJBc.rst b/Misc/NEWS.d/next/Tests/2018-07-10-18-53-46.bpo-0.UBQJBc.rst
new file mode 100644
index 0000000..9d82677
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2018-07-10-18-53-46.bpo-0.UBQJBc.rst
@@ -0,0 +1 @@
+Improved an error message when mock assert_has_calls fails.