diff --git a/dist/types/typeUtils/MergeDeep.d.ts b/dist/types/typeUtils/MergeDeep.d.ts
index 10e0f0e..3e679ff 100644
--- a/dist/types/typeUtils/MergeDeep.d.ts
+++ b/dist/types/typeUtils/MergeDeep.d.ts
@@ -21,9 +21,9 @@ declare type RequiredKeys<O extends object> = O extends unknown ? {
     [K in keyof O]-?: {} extends Pick<O, K> ? never : K;
 }[keyof O] : never;
 declare type MergeObjectDeeply<O extends Record<string | number | symbol, unknown>, O1 extends Record<string | number | symbol, unknown>> = {
-    [K in keyof (O & O1)]: K extends RequiredKeys<O1> ? O1[K] : K extends OptionalKeys<O1> ? K extends OptionalKeys<O> ? MergeObjectOrReturnUnion<Exclude<O[K], undefined>, Exclude<O1[K], undefined>> : K extends RequiredKeys<O> ? Exclude<O1[K], undefined> extends O[K] ? O[K] : MergeObjectOrReturnUnion<O[K], Exclude<O1[K], undefined>> : O1[K] : O[K];
+    [K in keyof (O & O1)]: K extends RequiredKeys<O1> ? MergeObjectsOrReturnFallback<O[K], O1[K], O1[K]> : K extends OptionalKeys<O1> ? K extends OptionalKeys<O> ? MergeObjectsOrReturnFallback<Exclude<O[K], undefined>, Exclude<O1[K], undefined>, Exclude<O[K], undefined> | Exclude<O1[K], undefined>> : K extends RequiredKeys<O> ? Exclude<O1[K], undefined> extends O[K] ? O[K] : MergeObjectsOrReturnFallback<O[K], Exclude<O1[K], undefined>, O[K] | Exclude<O1[K], undefined>> : O1[K] : O[K];
 };
-declare type MergeObjectOrReturnUnion<Val0, Val1> = Val0 extends Record<string | number | symbol, unknown> ? Val1 extends Record<string | number | symbol, unknown> ? MergeObjectDeeply<Val0, Val1> : Val0 | Val1 : Val0 | Val1;
+declare type MergeObjectsOrReturnFallback<O, O1, Fallback> = O extends Record<string | number | symbol, unknown> ? O1 extends Record<string | number | symbol, unknown> ? MergeObjectDeeply<O, O1> : Fallback : Fallback;
 /**
  * Accurately merge the fields of `O` with the ones of `O1`. It is
  * equivalent to the spread operator in JavaScript. [[Union]]s and [[Optional]]
@@ -35,31 +35,22 @@ declare type MergeObjectOrReturnUnion<Val0, Val1> = Val0 extends Record<string |
  * @returns [[Object]]
  * @example
  * ```ts
- * type O = {
- *   name?: string
- *   age?: number
- *   zip?: string
- *   pay: { cvv?: number }
- * }
+ * import { PrettyPrint } from './PrettyPrint'
  *
- * type O1 = {
- *   age: number
- *   zip?: number
- *   pay: { cvv : number; ccn?: string }
- *   city: string
- * }
+ * type A1 = { a: number; b?: number;            d?: number; e?: number; x: string;             y?: number; z: string;  } // prettier-ignore
+ * type A2 = { a?: number;           c?: number; d?: number; e: number;  x: number | undefined; y?: string; z?: number; } // prettier-ignore
  *
- * type test = MergeDeep<O, O1>
- * // {
- * //     name?: string;
- * //     age: number;
- * //     zip: number | undefined;
- * //     pay: {
- * //         cvv: number;
- * //         ccn?: string;
- * //     };
- * //     city: string;
- * // }
+ * type Result = PrettyPrint<MergeDeep<A1, A2>>
+ * {
+ *    a: number;
+ *    b?: number | undefined;
+ *    c?: number | undefined;
+ *    d?: number | undefined;
+ *    e: number;
+ *    x: number | undefined;
+ *    y?: string | number | undefined;
+ *    z: string | number;
+ * }
  * ```
  */
 export declare type MergeDeep<O extends Record<string | number | symbol, unknown>, O1 extends Record<string | number | symbol, unknown>> = O extends unknown ? (O1 extends unknown ? MergeObjectDeeply<O, O1> : never) : never;
diff --git a/src/typeUtils/MergeDeep.ts b/src/typeUtils/MergeDeep.ts
index a04fdf6..cb4f472 100644
--- a/src/typeUtils/MergeDeep.ts
+++ b/src/typeUtils/MergeDeep.ts
@@ -1,5 +1,3 @@
-import { PrettyPrint } from './PrettyPrint'
-
 /**
  * Get the keys of `O` that are optional
  * @param O
@@ -35,23 +33,34 @@ type MergeObjectDeeply<
   O1 extends Record<string | number | symbol, unknown>
 > = {
   [K in keyof (O & O1)]: K extends RequiredKeys<O1> // second prop is non-optional
-    ? O1[K] // return second prop
+    ? MergeObjectsOrReturnFallback<O[K], O1[K], O1[K]> // return second prop
     : K extends OptionalKeys<O1> // second prop is optional
     ? K extends OptionalKeys<O> // first prop is optional (second prop also)
-      ? MergeObjectOrReturnUnion<Exclude<O[K], undefined>, Exclude<O1[K], undefined>> // return union
+      ? MergeObjectsOrReturnFallback<
+          Exclude<O[K], undefined>,
+          Exclude<O1[K], undefined>,
+          Exclude<O[K], undefined> | Exclude<O1[K], undefined>
+        > // return union
       : K extends RequiredKeys<O> // first prop required (second prop optional)
       ? Exclude<O1[K], undefined> extends O[K] // (optional) second prop has the same type as the (required) first prop
         ? O[K] // return only the first one
-        : MergeObjectOrReturnUnion<O[K], Exclude<O1[K], undefined>> // (optional) second prop has a different type as the (required) first prop, so return union without `undefined` in the second
+        : MergeObjectsOrReturnFallback<
+            O[K],
+            Exclude<O1[K], undefined>,
+            O[K] | Exclude<O1[K], undefined>
+          > // (optional) second prop has a different type as the (required) first prop, so return union without `undefined` in the second
       : O1[K] // first prop inexistent, so return second prop
     : O[K] // second prop inexistent, so return first prop
 }
 
-type MergeObjectOrReturnUnion<Val0, Val1> = Val0 extends Record<string | number | symbol, unknown>
-  ? Val1 extends Record<string | number | symbol, unknown>
-    ? MergeObjectDeeply<Val0, Val1>
-    : Val0 | Val1
-  : Val0 | Val1
+type MergeObjectsOrReturnFallback<O, O1, Fallback> = O extends Record<
+  string | number | symbol,
+  unknown
+>
+  ? O1 extends Record<string | number | symbol, unknown>
+    ? MergeObjectDeeply<O, O1>
+    : Fallback
+  : Fallback
 
 /**
  * Accurately merge the fields of `O` with the ones of `O1`. It is
@@ -64,31 +73,22 @@ type MergeObjectOrReturnUnion<Val0, Val1> = Val0 extends Record<string | number
  * @returns [[Object]]
  * @example
  * ```ts
- * type O = {
- *   name?: string
- *   age?: number
- *   zip?: string
- *   pay: { cvv?: number }
- * }
+ * import { PrettyPrint } from './PrettyPrint'
  *
- * type O1 = {
- *   age: number
- *   zip?: number
- *   pay: { cvv : number; ccn?: string }
- *   city: string
- * }
+ * type A1 = { a: number; b?: number;            d?: number; e?: number; x: string;             y?: number; z: string;  } // prettier-ignore
+ * type A2 = { a?: number;           c?: number; d?: number; e: number;  x: number | undefined; y?: string; z?: number; } // prettier-ignore
  *
- * type test = MergeDeep<O, O1>
- * // {
- * //     name?: string;
- * //     age: number;
- * //     zip: number | undefined;
- * //     pay: {
- * //         cvv: number;
- * //         ccn?: string;
- * //     };
- * //     city: string;
- * // }
+ * type Result = PrettyPrint<MergeDeep<A1, A2>>
+ * {
+ *    a: number;
+ *    b?: number | undefined;
+ *    c?: number | undefined;
+ *    d?: number | undefined;
+ *    e: number;
+ *    x: number | undefined;
+ *    y?: string | number | undefined;
+ *    z: string | number;
+ * }
  * ```
  */
 export type MergeDeep<
@@ -96,32 +96,7 @@ export type MergeDeep<
   O1 extends Record<string | number | symbol, unknown>
 > = O extends unknown ? (O1 extends unknown ? MergeObjectDeeply<O, O1> : never) : never
 
-// type O = {
-//   name?: string
-//   age?: number
-//   zip?: string
-//   pay: { cvv?: number; xyz: RegExp }
-// }
-
-// type O1 = {
-//   age: number
-//   zip?: number
-//   pay: { cvv: number; ccn?: string }
-//   city: string
-// }
-
-// type test = MergeDeep<O, O1>
-// {
-//     name?: string;
-//     age: number;
-//     zip: number | undefined;
-//     pay: {
-//         cvv: number;
-//         ccn?: string;
-//     };
-//     city: string;
-// }
-
+// import { PrettyPrint } from './PrettyPrint'
 // type A1 = { arr: string[]; barr?: { b: number } }
 // type A2 = { arr?: number[]; barr?: { b: number } }
 // type TestA = PrettyPrint<MergeDeep<A1, A2>>
@@ -130,6 +105,10 @@ export type MergeDeep<
 // type B2 = { a?: number;           c?: number; d?: number; e: number;  x: number | undefined; y?: string; z?: number; } // prettier-ignore
 // type TestB = PrettyPrint<MergeDeep<B1, B2>>
 
+// type C1 = { info: { time: string; newDate: Date; very: { deep: { prop: boolean } } } }
+// type C2 = { info: { date: string; very: { deep: { prop: boolean } } } }
+// type TestC = PrettyPrint<MergeDeep<C1, C2>>
+
 // import { Timestamp } from 'firebase/firestore'
 // type T1 = { date: Timestamp }
 // type T2 = { date: Timestamp }