diff --git a/wiremock/server/server.py b/wiremock/server/server.py
index 88b39aa..b330f0c 100644
--- a/wiremock/server/server.py
+++ b/wiremock/server/server.py
@@ -61,7 +61,7 @@ def start(self):
             "--local-response-templating",
         ]
         if self.root_dir is not None:
-            cmd.append("--root-dir=")
+            cmd.append("--root-dir")
             cmd.append(str(self.root_dir))
         try:
             self.__subprocess = Popen(cmd, stdin=PIPE, stdout=PIPE, stderr=STDOUT)