diff --git a/crates/cli/src/bin/wasm-bindgen-test-runner/headless.rs b/crates/cli/src/bin/wasm-bindgen-test-runner/headless.rs
index ad6cc3a70900..62c61bad98f4 100644
--- a/crates/cli/src/bin/wasm-bindgen-test-runner/headless.rs
+++ b/crates/cli/src/bin/wasm-bindgen-test-runner/headless.rs
@@ -283,16 +283,21 @@ impl Client {
             Driver::Safari(_) => {
                 #[derive(Deserialize)]
                 struct Response {
-                    // returned by `--legacy`
+                    // returned by `--legacy` or by default on High Sierra and lower.
                     #[serde(rename = "sessionId")]
                     session_id: Option<String>,
                     // returned by the now-default `--w3c` mode
+                    #[serde(flatten)]
+                    #[allow(dead_code)]
                     value: Option<Value>,
                 }
                 #[derive(Deserialize)]
                 struct Value {
+                    // This needs to be optional because both `--legacy` and High Sierra do not
+                    // include a session id in the value entry.
                     #[serde(rename = "sessionId")]
-                    session_id: String,
+                    #[allow(dead_code)]
+                    session_id: Option<String>,
                 }
                 let request = json!({
                     // this is needed for the now `--legacy` mode
@@ -303,7 +308,7 @@ impl Client {
                     }
                 });
                 let x: Response = self.post("/session", &request)?;
-                Ok(x.session_id.or(x.value.map(|v| v.session_id)).unwrap())
+                Ok(x.session_id.unwrap())
             }
             Driver::Chrome(_) => {
                 #[derive(Deserialize)]