diff --git a/lib/gulp-eslint-new.d.ts b/lib/gulp-eslint-new.d.ts
index 2f4a4b5..f2f47b8 100644
--- a/lib/gulp-eslint-new.d.ts
+++ b/lib/gulp-eslint-new.d.ts
@@ -113,8 +113,9 @@ declare namespace gulpESLintNew
     }
 
     type GulpESLintOptions =
-    (GulpESLintrcOptions & { configType?: 'eslintrc' | null | undefined; }) |
-    (GulpFlatESLintOptions & { configType: 'flat'; });
+    | ((GulpESLintrcOptions | GulpFlatESLintOptions) & { configType?: null | undefined; })
+    | (GulpESLintrcOptions & { configType: 'eslintrc'; })
+    | (GulpFlatESLintOptions & { configType: 'flat'; });
 
     type GulpESLintResult = eslint.LintResult;
 
diff --git a/test/ts-defs-test.ts b/test/ts-defs-test.ts
index fc71f5b..e9af687 100644
--- a/test/ts-defs-test.ts
+++ b/test/ts-defs-test.ts
@@ -26,6 +26,25 @@ gulpESLintNew
     },
 );
 
+{
+    let configType!: undefined | null;
+    gulpESLintNew
+    ({ configType, ignorePatterns: undefined, ruleFilter: undefined, stats: undefined });
+}
+
+{
+    let configType!: undefined | null;
+    gulpESLintNew
+    (
+        {
+            configType,
+            reportUnusedDisableDirectives:  undefined,
+            resolvePluginsRelativeTo:       undefined,
+            rulePaths:                      undefined,
+        },
+    );
+}
+
 gulpESLintNew
 (
     {