diff --git a/src/test/conversion-union.test.ts b/src/test/conversion-union.test.ts
new file mode 100644
index 0000000..e6e6f37
--- /dev/null
+++ b/src/test/conversion-union.test.ts
@@ -0,0 +1,37 @@
+import type {FilterSetConfig} from '../types'
+import {convertFilterSetConfig} from '../middleware'
+
+interface SpecialNumber {
+  internal: number
+}
+
+interface Data {
+  number: number | SpecialNumber
+}
+
+test('it should convert a primitive of a union type', () => {
+  const simpleConfig: FilterSetConfig<Data> = {
+    number: {lt: 123},
+  }
+  const converted = convertFilterSetConfig(simpleConfig)
+  // eslint-disable-next-line camelcase
+  expect(converted).toEqual({number__lt: 123})
+})
+
+test('it should convert a complex type of a union type', () => {
+  const special = {internal: 123}
+  const simpleConfig: FilterSetConfig<Data> = {
+    number: {value: special},
+  }
+  const converted = convertFilterSetConfig(simpleConfig)
+  expect(converted).toEqual({number: special})
+})
+
+test('the config should be able to filter attributes of complex types', () => {
+  const simpleConfig: FilterSetConfig<Data> = {
+    number: {internal: {value: 123}},
+  }
+  const converted = convertFilterSetConfig(simpleConfig)
+  // eslint-disable-next-line camelcase
+  expect(converted).toEqual({number__internal: 123})
+})
diff --git a/src/types.ts b/src/types.ts
index d926cf4..51d7099 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -120,7 +120,7 @@ export type FilterSetConfig<D = Record<any, any>, K extends FSKeyConfig<D> | nul
       : CheckConfigKeys<D, Exclude<K, null>, key, C> // check if key is inside the config
   ) | (
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    D[key] extends (Record<any, any> | undefined) ? // check if the type of the key is a Record, so we add extended references
+    Extract<object, D[key]> extends (Record<any, any> | undefined) ? // check if the type of the key is a Record, so we add extended references
         (key extends keyof K ?
           FilterSetConfig<D[key], K[key]>
           : FilterSetConfig<D[key]>)