diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -92,6 +92,13 @@
         exec(c)
         self.assertEqual(x[0], 'foo3')
 
+    def test_compile_time_concat_errors(self):
+        self.assertAllRaise(SyntaxError,
+                            'cannot mix bytes and nonbytes literals',
+                            [r"""f'' b''""",
+                             r"""b'' f''""",
+                             ])
+
     def test_literal(self):
         self.assertEqual(f'', '')
         self.assertEqual(f'a', 'a')
diff --git a/Python/ast.c b/Python/ast.c
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -5147,7 +5147,8 @@
         /* Check that we're not mixing bytes with unicode. */
         if (i != 0 && bytesmode != this_bytesmode) {
             ast_error(c, n, "cannot mix bytes and nonbytes literals");
-            Py_DECREF(s);
+            /* s is NULL if the current string part is an f-string. */
+            Py_XDECREF(s);
             goto error;
         }
         bytesmode = this_bytesmode;