--- a/testing/webdriver/src/capabilities.rs
+++ b/testing/webdriver/src/capabilities.rs
@@ -88,17 +88,17 @@ impl SpecNewSessionParameters {
                 "acceptInsecureCerts" => if !value.is_boolean() {
                         return Err(WebDriverError::new(ErrorStatus::InvalidArgument,
                                                        "acceptInsecureCerts was not a boolean"))
                     },
                 x @ "browserName" |
                 x @ "browserVersion" |
                 x @ "platformName" => if !value.is_string() {
                         return Err(WebDriverError::new(ErrorStatus::InvalidArgument,
-                                                       format!("{} was not a boolean", x)))
+                                                       format!("{} is not a string", x)))
                     },
                 "pageLoadStrategy" => {
                     try!(SpecNewSessionParameters::validate_page_load_strategy(value))
                 }
                 "proxy" => {
                     try!(SpecNewSessionParameters::validate_proxy(value))
                 },
                 "timeouts" => {