diff --git a/src/marionette/message.rs b/src/marionette/message.rs
index aba51f7..00d3a85 100644
--- a/src/marionette/message.rs
+++ b/src/marionette/message.rs
@@ -41,7 +41,7 @@ pub struct Capabilities {
     pub webdriver_click: bool,
     pub page_load_strategy: String,
     pub platform_name: String,
-    pub platform_version: String,
+    pub platform_version: Option<String>,
     // proxy:
     pub set_window_rect: bool,
     pub timeouts: Timeouts,