diff --git a/src/utils.ts b/src/utils.ts
index 2160209..21b7d71 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -2,7 +2,7 @@ import { errorCodes, errorValues } from './error-constants';
 import { EthereumRpcError, SerializedEthereumRpcError } from './classes';
 
 const FALLBACK_ERROR_CODE = errorCodes.rpc.internal;
-const FALLBACK_MESSAGE = 'Unspecified error message. This is a bug, please report it.';
+const FALLBACK_MESSAGE = 'Invalid internal error. See "data.originalError" for original value. Please report this bug.';
 const FALLBACK_ERROR: SerializedEthereumRpcError = {
   code: FALLBACK_ERROR_CODE,
   message: getMessageFromCode(FALLBACK_ERROR_CODE),
@@ -20,7 +20,7 @@ export function getMessageFromCode(
   code: number,
   fallbackMessage: string = FALLBACK_MESSAGE,
 ): string {
-  if (Number.isInteger(code)) {
+  if (isValidCode(code)) {
     const codeString = code.toString();
 
     if (hasKey(errorValues, codeString)) {
@@ -35,22 +35,10 @@ export function getMessageFromCode(
 
 /**
  * Returns whether the given code is valid.
- * A code is only valid if it has a message.
+ * A code is valid if it is an integer number.
  */
 export function isValidCode(code: number): boolean {
-  if (!Number.isInteger(code)) {
-    return false;
-  }
-
-  const codeString = code.toString();
-  if (errorValues[codeString as ErrorValueKey]) {
-    return true;
-  }
-
-  if (isJsonRpcServerError(code)) {
-    return true;
-  }
-  return false;
+  return Number.isInteger(code);
 }
 
 /**
diff --git a/test/serializeError.js b/test/serializeError.js
index 1fee3fb..24b037c 100644
--- a/test/serializeError.js
+++ b/test/serializeError.js
@@ -11,12 +11,12 @@ const dummyMessage = 'baz';
 
 const invalidError0 = 0;
 const invalidError1 = ['foo', 'bar', 3];
-const invalidError2 = { code: 34 };
+const invalidError2 = { code: 'Invalid code, should be an integer' };
 const invalidError3 = { code: 4001 };
 const invalidError4 = { code: 4001, message: 3, data: Object.assign({}, dummyData) };
 const invalidError5 = null;
 const invalidError6 = undefined;
-const invalidError7 = { code: 34, message: dummyMessage, data: Object.assign({}, dummyData) };
+const invalidError7 = { code: 'Invalid code, should be an integer', message: dummyMessage, data: Object.assign({}, dummyData) };
 
 const validError0 = { code: 4001, message: dummyMessage };
 const validError1 = { code: 4001, message: dummyMessage, data: Object.assign({}, dummyData) };