diff --git a/setup.py b/setup.py
index d58f07fd..0e083b5d 100644
--- a/setup.py
+++ b/setup.py
@@ -56,6 +56,7 @@ def get_long_description():
             else ""
         ),
         "pystrict",
+        "flexparser<0.4",
     ],
     extras_require={
         "tests": [