diff --git a/requests_cache/_utils.py b/requests_cache/_utils.py
index eaaa7e53..22be04e0 100644
--- a/requests_cache/_utils.py
+++ b/requests_cache/_utils.py
@@ -7,7 +7,7 @@
 
 from urllib3 import filepost
 
-FORM_BOUNDARY = '==requests-cache-form-boundary=='
+FORM_BOUNDARY = '--requests-cache-form-boundary--'
 
 KwargDict = Dict[str, Any]
 logger = getLogger('requests_cache')