diff --git a/src/index.ts b/src/index.ts
index 7b4cca9..33025e1 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -13,6 +13,10 @@ interface RpcRequest {
   params?: Record<string, unknown>;
 }
 
+function isRpcRequestObject(value: unknown): value is RpcRequest {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}
+
 export default {
   async fetch(req: Request, env: Env): Promise<Response> {
     const url = new URL(req.url);
@@ -51,12 +55,15 @@ export default {
       return aiChat(req, env);
     }
 
-    let body: RpcRequest;
+    let body: unknown;
     try {
-      body = (await req.json()) as RpcRequest;
+      body = await req.json();
     } catch {
       return json(rpcError(null, -32700, "parse error"));
     }
+    if (!isRpcRequestObject(body)) {
+      return json(rpcError(null, -32600, "invalid request"));
+    }
 
     const result = await handle(body, env, caller);
     // Notifications (no id) get a 202 with no body per JSON-RPC.
diff --git a/test/index.test.mjs b/test/index.test.mjs
index ef49ed2..28fee1c 100644
--- a/test/index.test.mjs
+++ b/test/index.test.mjs
@@ -69,6 +69,26 @@ function mcpRequest(env, body) {
   );
 }
 
+test("rejects non-object JSON-RPC request bodies", async (t) => {
+  const env = {
+    BRIDGEKIT_CLIENTS: JSON.stringify({
+      "client-key": { name: "reader", tools: [], allowWrite: false },
+    }),
+  };
+
+  for (const body of [null, [], "ping", 42, true]) {
+    await t.test(JSON.stringify(body), async () => {
+      const response = await mcpRequest(env, body);
+
+      assert.deepEqual(await response.json(), {
+        jsonrpc: "2.0",
+        id: null,
+        error: { code: -32600, message: "invalid request" },
+      });
+    });
+  }
+});
+
 test("tools/call rejects non-object arguments", async (t) => {
   let upstreamCalls = 0;
   globalThis.fetch = async () => {