diff --git a/src/json.test.ts b/src/json.test.ts
index dc947c09..817e37fa 100644
--- a/src/json.test.ts
+++ b/src/json.test.ts
@@ -1,4 +1,5 @@
 import * as superstructModule from 'superstruct';
+import { validate } from 'superstruct';
 import {
   ARRAY_OF_DIFFRENT_KINDS_OF_NUMBERS,
   COMPLEX_OBJECT,
@@ -13,6 +14,8 @@ import {
   NON_SERIALIZABLE_NESTED_OBJECT,
 } from './__fixtures__';
 import {
+  assert,
+  assertIsJsonRpcError,
   assertIsJsonRpcFailure,
   assertIsJsonRpcNotification,
   assertIsJsonRpcRequest,
@@ -20,6 +23,7 @@ import {
   assertIsJsonRpcSuccess,
   assertIsPendingJsonRpcResponse,
   getJsonRpcIdValidator,
+  isJsonRpcError,
   isJsonRpcFailure,
   isJsonRpcNotification,
   isJsonRpcRequest,
@@ -27,12 +31,21 @@ import {
   isJsonRpcSuccess,
   isPendingJsonRpcResponse,
   isValidJson,
+  JsonStruct,
   validateJsonAndGetSize,
-  isJsonRpcError,
-  assertIsJsonRpcError,
 } from '.';
 
 describe('json', () => {
+  describe('JsonStruct', () => {
+    it('returns error message', () => {
+      const [error] = validate(undefined, JsonStruct);
+      assert(error !== undefined);
+      expect(error.message).toStrictEqual(
+        'Expected a valid JSON-serializable value',
+      );
+    });
+  });
+
   // TODO: Make this test suite exhaustive.
   // The current implementation is guaranteed to be correct, but in the future
   // we may opt for a bespoke implementation that is more performant, but may
@@ -425,11 +438,11 @@ describe('json', () => {
     };
 
     const validateAll = (
-      validate: ReturnType<typeof getJsonRpcIdValidator>,
+      validator: ReturnType<typeof getJsonRpcIdValidator>,
       inputs: ReturnType<typeof getInputs>,
     ) => {
       for (const input of Object.values(inputs)) {
-        expect(validate(input.value)).toStrictEqual(input.expected);
+        expect(validator(input.value)).toStrictEqual(input.expected);
       }
     };
 
diff --git a/src/json.ts b/src/json.ts
index 7847b932..745d0ea8 100644
--- a/src/json.ts
+++ b/src/json.ts
@@ -10,22 +10,26 @@ import {
   object,
   omit,
   optional,
+  record,
   string,
   Struct,
   union,
   unknown,
 } from 'superstruct';
+import { AssertionErrorConstructor, assertStruct } from './assert';
 import {
   calculateNumberSize,
   calculateStringSize,
   isPlainObject,
   JsonSize,
 } from './misc';
-import { AssertionErrorConstructor, assertStruct } from './assert';
 
 export const JsonStruct = define<Json>('Json', (value) => {
   const [isValid] = validateJsonAndGetSize(value, true);
-  return isValid;
+  if (!isValid) {
+    return 'Expected a valid JSON-serializable value';
+  }
+  return true;
 });
 
 /**
@@ -99,8 +103,9 @@ export type JsonRpcError = OptionalField<
   'data'
 >;
 
-export const JsonRpcParamsStruct = optional(union([object(), array()]));
-
+export const JsonRpcParamsStruct = optional(
+  union([record(string(), JsonStruct), array(JsonStruct)]),
+);
 export type JsonRpcParams = Infer<typeof JsonRpcParamsStruct>;
 
 export const JsonRpcRequestStruct = object({