diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 1dab166..4ed37ea 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -781,6 +781,22 @@ class IOTest(unittest.TestCase):
         with self.open("non-existent", "r", opener=opener) as f:
             self.assertEqual(f.read(), "egg\n")
 
+    def test_bad_opener_negative_1(self):
+        # Issue #27066.
+        def badopener(fname, flags):
+            return -1
+        with self.assertRaises(ValueError) as cm:
+            open('non-existent', 'r', opener=badopener)
+        self.assertEqual(str(cm.exception), 'opener returned -1')
+
+    def test_bad_opener_other_negative(self):
+        # Issue #27066.
+        def badopener(fname, flags):
+            return -2
+        with self.assertRaises(ValueError) as cm:
+            open('non-existent', 'r', opener=badopener)
+        self.assertEqual(str(cm.exception), 'opener returned -2')
+
     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..8b83372 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -420,7 +420,11 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
 
             self->fd = _PyLong_AsInt(fdobj);
             Py_DECREF(fdobj);
-            if (self->fd == -1) {
+            if (self->fd < 0) {
+                if (!PyErr_Occurred())
+                    /* The opener returned -1.  See issue #27066 */
+                    PyErr_Format(PyExc_ValueError,
+                                 "opener returned %d", self->fd);
                 goto error;
             }
         }