diff --git a/src/lib/merge-configs.ts b/src/lib/merge-configs.ts
index 8eb947a8..df2d765f 100644
--- a/src/lib/merge-configs.ts
+++ b/src/lib/merge-configs.ts
@@ -4,7 +4,7 @@ import { ConfigExtension, GenericConfig } from './types'
  * @param baseConfig Config where other config will be merged into. This object will be mutated.
  * @param configExtension Partial config to merge into the `baseConfig`.
  */
-export function mergeConfigs<ClassGroupIds extends string, ThemeGroupIds extends string>(
+export function mergeConfigs<ClassGroupIds extends string, ThemeGroupIds extends string = never>(
     baseConfig: GenericConfig,
     {
         cacheSize,
diff --git a/tests/type-generics.test.ts b/tests/type-generics.test.ts
index c0993b29..07a46e59 100644
--- a/tests/type-generics.test.ts
+++ b/tests/type-generics.test.ts
@@ -169,4 +169,8 @@ test('mergeConfigs type generics work correctly', () => {
     const config2 = mergeConfigs<'very', 'strict'>(getDefaultConfig(), {})
 
     expect(config2).toEqual(expect.any(Object))
+
+    const config3 = mergeConfigs<'single-arg'>(getDefaultConfig(), {})
+
+    expect(config3).toEqual(expect.any(Object))
 })