diff --git a/src/jrpc/errors/utils.ts b/src/jrpc/errors/utils.ts
index 2f61f8f1..abd5300d 100644
--- a/src/jrpc/errors/utils.ts
+++ b/src/jrpc/errors/utils.ts
@@ -9,6 +9,19 @@ declare type PropertyKey = string | number | symbol;
 
 type ErrorValueKey = keyof typeof errorValues;
 
+export function isValidNumber(value: unknown): value is number {
+  try {
+    if (typeof value === "number" && Number.isInteger(value)) {
+      return true;
+    }
+
+    const parsedValue = Number(value.toString());
+    return Number.isInteger(parsedValue);
+  } catch {
+    return false;
+  }
+}
+
 /**
  * Returns whether the given code is valid.
  * A code is valid if it is an integer.
diff --git a/src/jrpc/jrpc.ts b/src/jrpc/jrpc.ts
index 08b95096..66171f42 100644
--- a/src/jrpc/jrpc.ts
+++ b/src/jrpc/jrpc.ts
@@ -1,5 +1,6 @@
 import { Duplex } from "readable-stream";
 
+import { errorCodes } from "./errors";
 import { AsyncJRPCMiddleware, ConsoleLike, IdMap, JRPCMiddleware, JRPCRequest, JRPCResponse, Json, ReturnHandlerCallback } from "./interfaces";
 import { SafeEventEmitter } from "./safeEventEmitter";
 import { SerializableError } from "./serializableError";
@@ -21,7 +22,7 @@ export function createErrorMiddleware(log: ConsoleLike): JRPCMiddleware<unknown,
     try {
       // json-rpc-engine will terminate the request when it notices this error
       if (typeof req.method !== "string" || !req.method) {
-        res.error = new SerializableError({ code: -32603, message: "invalid method" });
+        res.error = new SerializableError({ code: errorCodes.rpc.invalidRequest, message: "invalid method" });
         end();
         return;
       }
@@ -35,7 +36,7 @@ export function createErrorMiddleware(log: ConsoleLike): JRPCMiddleware<unknown,
       });
     } catch (error: unknown) {
       log.error(`Auth - RPC Error thrown: ${(error as Error).message}`, error);
-      res.error = new SerializableError({ code: -32603, message: (error as Error).message });
+      res.error = new SerializableError({ code: errorCodes.rpc.internal, message: (error as Error).message });
       end();
     }
   };
diff --git a/src/jrpc/jrpcEngine.ts b/src/jrpc/jrpcEngine.ts
index 8fd3fb93..2127a11c 100644
--- a/src/jrpc/jrpcEngine.ts
+++ b/src/jrpc/jrpcEngine.ts
@@ -2,7 +2,7 @@ import { Duplex } from "readable-stream";
 
 import { log } from "../utils/logger";
 import { errorCodes, JsonRpcError } from "./errors";
-import { getMessageFromCode, serializeJrpcError } from "./errors/utils";
+import { getMessageFromCode, isValidNumber, serializeJrpcError } from "./errors/utils";
 import {
   JRPCEngineEndCallback,
   JRPCEngineNextCallback,
@@ -25,11 +25,11 @@ export type JrpcEngineEvents = {
 function constructFallbackError(error: Error): JRPCError {
   const {
     message = "",
-    code = -32603,
+    code = errorCodes.rpc.internal,
     stack = "Stack trace is not available.",
     data = "",
   } = error as { message?: string; code?: number; stack?: string; data?: string };
-  const codeNumber = parseInt(code?.toString() || "-32603");
+  const codeNumber = isValidNumber(code) ? parseInt(code.toString()) : errorCodes.rpc.internal;
   return {
     message: message || error?.toString() || getMessageFromCode(codeNumber),
     code: codeNumber,
@@ -116,7 +116,7 @@ export class JRPCEngine extends SafeEventEmitter<JrpcEngineEvents> {
         } else {
           if (returnHandler) {
             if (typeof returnHandler !== "function") {
-              end(new SerializableError({ code: -32603, message: "JRPCEngine: 'next' return handlers must be functions" }));
+              end(new SerializableError({ code: errorCodes.rpc.internal, message: "JRPCEngine: 'next' return handlers must be functions" }));
             }
             returnHandlers.push(returnHandler);
           }
@@ -152,10 +152,10 @@ export class JRPCEngine extends SafeEventEmitter<JrpcEngineEvents> {
    */
   private static _checkForCompletion(_req: JRPCRequest<unknown>, res: JRPCResponse<unknown>, isComplete: boolean): void {
     if (!("result" in res) && !("error" in res)) {
-      throw new SerializableError({ code: -32603, message: "Response has no error or result for request" });
+      throw new SerializableError({ code: errorCodes.rpc.internal, message: "Response has no error or result for request" });
     }
     if (!isComplete) {
-      throw new SerializableError({ code: -32603, message: "Nothing ended request" });
+      throw new SerializableError({ code: errorCodes.rpc.internal, message: "Nothing ended request" });
     }
   }
 
@@ -268,6 +268,17 @@ export class JRPCEngine extends SafeEventEmitter<JrpcEngineEvents> {
   ): Promise<JRPCResponse<unknown>[] | void> {
     // The order here is important
     try {
+      if (reqs.length === 0) {
+        const error = new SerializableError({
+          code: errorCodes.rpc.invalidRequest,
+          message: "Request batch must contain plain objects. Received an empty array",
+        });
+        const response: JRPCResponse<unknown>[] = [{ id: undefined, jsonrpc: "2.0" as const, error }];
+        if (cb) {
+          return cb(error, response);
+        }
+        return response;
+      }
       // 2. Wait for all requests to finish, or throw on some kind of fatal
       // error
       const responses = await Promise.all(
@@ -312,12 +323,18 @@ export class JRPCEngine extends SafeEventEmitter<JrpcEngineEvents> {
    */
   private async _handle(callerReq: JRPCRequest<unknown>, cb: (error: unknown, response: JRPCResponse<unknown>) => void): Promise<void> {
     if (!callerReq || Array.isArray(callerReq) || typeof callerReq !== "object") {
-      const error = new SerializableError({ code: -32603, message: "request must be plain object" });
+      const error = new SerializableError({
+        code: errorCodes.rpc.invalidRequest,
+        message: `Requests must be plain objects. Received: ${typeof callerReq}`,
+      });
       return cb(error, { id: undefined, jsonrpc: "2.0", error });
     }
 
-    if (typeof callerReq.method !== "string") {
-      const error = new SerializableError({ code: -32603, message: "method must be string" });
+    if (typeof callerReq.method !== "string" || !callerReq.method) {
+      const error = new SerializableError({
+        code: errorCodes.rpc.invalidRequest,
+        message: `Must specify a string method. Received: ${typeof callerReq.method}`,
+      });
       return cb(error, { id: callerReq.id, jsonrpc: "2.0", error });
     }
 
diff --git a/test/jrpcEngine.test.ts b/test/jrpcEngine.test.ts
new file mode 100644
index 00000000..b9e6e4ca
--- /dev/null
+++ b/test/jrpcEngine.test.ts
@@ -0,0 +1,33 @@
+import { describe, expect, it } from "vitest";
+
+import { errorCodes } from "../src/jrpc/errors";
+import { JRPCEngine } from "../src/jrpc/jrpcEngine";
+
+describe("JRPCEngine request validation", () => {
+  it("returns invalidRequest for non-object requests", async () => {
+    const engine = new JRPCEngine();
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const response = await engine.handle(123 as any);
+    expect(response.error?.code).toBe(errorCodes.rpc.invalidRequest);
+    expect(response.id).toBeUndefined();
+    expect(response.jsonrpc).toBe("2.0");
+  });
+
+  it("returns invalidRequest for non-string method", async () => {
+    const engine = new JRPCEngine();
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const response = await engine.handle({ id: 1, jsonrpc: "2.0", method: 123 as any });
+    expect(response.error?.code).toBe(errorCodes.rpc.invalidRequest);
+    expect(response.id).toBe(1);
+    expect(response.jsonrpc).toBe("2.0");
+  });
+
+  it("returns invalidRequest for empty batch requests", async () => {
+    const engine = new JRPCEngine();
+    const responses = await engine.handle([]);
+    expect(responses).toHaveLength(1);
+    expect(responses[0].error?.code).toBe(errorCodes.rpc.invalidRequest);
+    expect(responses[0].id).toBeUndefined();
+    expect(responses[0].jsonrpc).toBe("2.0");
+  });
+});