diff --git a/esrally/client/common.py b/esrally/client/common.py
index dc6ec0bb7..e0ca051c9 100644
--- a/esrally/client/common.py
+++ b/esrally/client/common.py
@@ -20,7 +20,7 @@ def _client_major_version_to_str(version: tuple) -> str:
 def _mimetype_header_to_compat(header, request_headers):
     # Converts all parts of a Accept/Content-Type headers
     # from application/X -> application/vnd.elasticsearch+X
-    mimetype = request_headers.get(header, None)
+    mimetype = request_headers.get(header, None) if request_headers else None
     if mimetype:
         request_headers[header] = _COMPAT_MIMETYPE_RE.sub(_COMPAT_MIMETYPE_SUB, mimetype)