diff --git a/requirements.txt b/requirements.txt
index 69c78180fc..6896abf147 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -25,7 +25,8 @@ python-socketio<5
 
 # esptool requirements (see components/esptool_py/esptool/setup.py)
 reedsolo>=1.5.3,<=1.5.4
-bitstring>=3.1.6
+# bitstring 4 dropped support for Python 3.6 and older
+bitstring>=3.1.6,<4
 ecdsa>=0.16.0
 
 # windows-curses are required in Windows command line but cannot be installed in MSYS2. A requirement like