diff --git a/synapse/app/synctl.py b/synapse/app/synctl.py
index 669a3ae92..39f4bf6e5 100755
--- a/synapse/app/synctl.py
+++ b/synapse/app/synctl.py
@@ -69,7 +69,7 @@ def main():
     cache_factor = config.get("synctl_cache_factor", None)
 
     if cache_factor:
-        os.environ["SYNAPSE_CACHE_FACTOR"] = cache_factor
+        os.environ["SYNAPSE_CACHE_FACTOR"] = str(cache_factor)
 
     action = sys.argv[1] if sys.argv[1:] else "usage"
     if action == "start":