diff --git a/packages/experimental-utils/src/ts-eslint/RuleTester.ts b/packages/experimental-utils/src/ts-eslint/RuleTester.ts
index 9a255b24207..65184298969 100644
--- a/packages/experimental-utils/src/ts-eslint/RuleTester.ts
+++ b/packages/experimental-utils/src/ts-eslint/RuleTester.ts
@@ -1,7 +1,7 @@
 import { RuleTester as ESLintRuleTester } from 'eslint';
 import { AST_NODE_TYPES, AST_TOKEN_TYPES } from '../ts-estree';
-import { ParserOptions } from './ParserOptions';
 import { Linter } from './Linter';
+import { ParserOptions } from './ParserOptions';
 import {
   RuleCreateFunction,
   RuleModule,
@@ -108,7 +108,7 @@ interface TestCaseError<TMessageIds extends string> {
   /**
    * Reported suggestions.
    */
-  readonly suggestions?: SuggestionOutput<TMessageIds>[] | null;
+  readonly suggestions?: readonly SuggestionOutput<TMessageIds>[] | null;
   /**
    * The type of the reported AST node.
    */