diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 5584d6b13ee..8581865145c 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -809,6 +809,22 @@ def opener(path, flags):
         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/Misc/NEWS b/Misc/NEWS
index 56ff6c82311..a76d229ce1f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@ What's New in Python 3.6.0 alpha 2
 Core and Builtins
 -----------------
 
+- Issue #27066: Fixed SystemError if a custom opener (for open()) returns a
+  negative number without setting an exception.
+
 - Issue #26983: float() now always return an instance of exact float.
   The deprecation warning is emitted if __float__ returns an instance of
   a strict subclass of float.  In a future versions of Python this can
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index fe6bc41a0f7..12e5156fbb7 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -420,7 +420,13 @@ _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 a negative but didn't set an
+                       exception.  See issue #27066 */
+                    PyErr_Format(PyExc_ValueError,
+                                 "opener returned %d", self->fd);
+                }
                 goto error;
             }
         }