[autofix.ci] apply automated fixes

Enhanced the regression test to document the difference between Node.js
and Bun error messages, and added a test case to ensure custom
prepareStackTrace functions continue to work as expected.

src/bun.js/bindings/ZigGlobalObject.cpp

@@ -2645,6 +2645,11 @@ JSC_DEFINE_HOST_FUNCTION(jsFunctionDefaultErrorPrepareStackTrace, (JSGlobalObjec
         return {};
     }
 
+    if (!callSites) {
+        throwTypeError(lexicalGlobalObject, scope, "Second argument must be an array of call sites"_s);
+        return {};
+    }
+
     JSValue result = formatStackTraceToJSValue(vm, globalObject, lexicalGlobalObject, errorObject, callSites, jsUndefined());
 
     RETURN_IF_EXCEPTION(scope, {});

test/regression/issue/21815.test.ts

@@ -0,0 +1,60 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+test("Error.prepareStackTrace called without second parameter should not crash", async () => {
+  // This test is for issue #21815
+  // The command `Error.prepareStackTrace(e)` was causing a segmentation fault
+  // due to missing validation of the second parameter (callSites array)
+  //
+  // Node.js throws: "Cannot read properties of undefined (reading 'length')"
+  // Bun now throws: "Second argument must be an array of call sites" (clearer message)
+
+  const code = `
+const e = new Error();
+try {
+  Error.prepareStackTrace(e);
+} catch (err) {
+  console.log("Caught error:", err.message);
+  process.exit(0);
+}
+console.log("No error thrown - this is unexpected");
+process.exit(1);
+  `;
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", code],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // The command should execute successfully without crashing
+  expect(exitCode).toBe(0);
+  expect(stdout).toContain("Caught error:");
+  expect(stdout).toContain("Second argument must be an array of call sites");
+  expect(stderr).toBe("");
+});
+
+test("Error.prepareStackTrace with custom function should work like Node.js", async () => {
+  // Custom prepareStackTrace functions should work normally, even with missing parameters
+  const code = `
+Error.prepareStackTrace = (err, stack) => "Custom: " + err.message;
+const e = new Error("test");
+console.log("Result:", Error.prepareStackTrace(e)); // Missing second param should be OK
+  `;
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", code],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(exitCode).toBe(0);
+  expect(stdout).toContain("Result: Custom: test");
+  expect(stderr).toBe("");
+});