diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -973,7 +973,10 @@
                 raise effect
 
             if not _callable(effect):
-                result = next(effect)
+                try:
+                    result = next(effect)
+                except StopIteration:
+                    result = None
                 if _is_exception(result):
                     raise result
                 if result is DEFAULT:
diff --git a/Lib/unittest/test/testmock/testmock.py b/Lib/unittest/test/testmock/testmock.py
--- a/Lib/unittest/test/testmock/testmock.py
+++ b/Lib/unittest/test/testmock/testmock.py
@@ -1419,6 +1419,15 @@
         self.assertEqual('abc', first)
         self.assertEqual('abc', second)
 
+    def test_mock_open_after_eof(self):
+        # read, readline and readlines should work after end of file.
+        _open = mock.mock_open(read_data='foo')
+        h = _open('bar')
+        h.read()
+        self.assertEqual('', h.read())
+        self.assertEqual('', h.readline())
+        self.assertEqual([], h.readlines())
+
     def test_mock_parents(self):
         for Klass in Mock, MagicMock:
             m = Klass()
diff --git a/Misc/ACKS b/Misc/ACKS
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1222,6 +1222,7 @@
 Mark Roberts
 Andy Robinson
 Jim Robinson
+Yolanda Robla
 Daniel Rocco
 Mark Roddy
 Kevin Rodgers
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -107,6 +107,9 @@
 Library
 -------
 
+- Issue #26807: mock_open 'files' no longer error on readline at end of file.
+  Patch from Yolanda Robla.
+
 - Issue #26837: assertSequenceEqual() now correctly outputs non-stringified
   differing items (like bytes in the -b mode).  This affects assertListEqual()
   and assertTupleEqual().