diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 1dab166..1f01670 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -781,6 +781,13 @@ class IOTest(unittest.TestCase):
         with self.open("non-existent", "r", opener=opener) as f:
             self.assertEqual(f.read(), "egg\n")
 
+    def test_bad_opener(self):
+        # Issue #27066
+        def badopener(fname, flags):
+            return -1
+        self.assertRaises(OSError, open, 'non-existent', 'r',
+                          opener=badopener)
+
     def test_fileio_closefd(self):
         # Issue #4841
         with self.open(__file__, 'rb') as f1, \
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index a02a9c1..539ff23 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -421,6 +421,9 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
             self->fd = _PyLong_AsInt(fdobj);
             Py_DECREF(fdobj);
             if (self->fd == -1) {
+                if (!PyErr_Occurred())
+                    /* The opener returned -1.  See issue #27066 */
+                    PyErr_SetString(PyExc_OSError, "opener returned -1");
                 goto error;
             }
         }