diff --git a/Lib/fileinput.py b/Lib/fileinput.py
--- a/Lib/fileinput.py
+++ b/Lib/fileinput.py
@@ -321,7 +321,7 @@
             if self._filename == '-':
                 self._filename = '<stdin>'
                 if 'b' in self._mode:
-                    self._file = sys.stdin.buffer
+                    self._file = getattr(sys.stdin, 'buffer', sys.stdin)
                 else:
                     self._file = sys.stdin
                 self._isstdin = True
diff --git a/Lib/test/test_fileinput.py b/Lib/test/test_fileinput.py
--- a/Lib/test/test_fileinput.py
+++ b/Lib/test/test_fileinput.py
@@ -240,6 +240,17 @@
             lines = list(fi)
             self.assertEqual(lines, [b'spam, bacon, sausage, and spam'])
 
+    def test_detached_stdin_binary_mode(self):
+        orig_stdin = sys.stdin
+        try:
+            sys.stdin = BytesIO(b'spam, bacon, sausage, and spam')
+            self.assertFalse(hasattr(sys.stdin, 'buffer'))
+            fi = FileInput(files=['-'], mode='rb')
+            lines = list(fi)
+            self.assertEqual(lines, [b'spam, bacon, sausage, and spam'])
+        finally:
+            sys.stdin = orig_stdin
+
     def test_file_opening_hook(self):
         try:
             # cannot use openhook and inplace mode