fix: Throw ArgumentError if not flat

lib/src/flatten.dart

@@ -47,4 +47,4 @@ Map<String, dynamic> flatten(
 }
 
 Map<String, T> _listToMap<T>(List<T> list) =>
-    list.asMap().map((key, value) => MapEntry(key.toString(), value));
+    list.asMap().map((key, value) => MapEntry(key.toString(), value));

lib/src/unflatten.dart

@@ -3,12 +3,25 @@
 /// If no [delimiter] is specified, will separate depth levels by `.`.
 ///
 /// Unflattens arrays given that the keys are integers.
+///
+/// Throws [ArgumentError] if any key is already a Map or List.
+///
+/// Throws [ArgumentError] if there are conflicting keys.
 Map<String, dynamic> unflatten(
   Map<String, dynamic> flatMap, {
   String delimiter = ".",
 }) {
   final Map<String, dynamic> result = {};
 
+  flatMap.forEach((key, value) {
+    if (value is Map) {
+      throw ArgumentError('Expected flat map, but key "$key" is a Map');
+    }
+    if (value is List) {
+      throw ArgumentError('Expected flat map, but key "$key" is a List');
+    }
+  });
+
   flatMap.forEach((key, value) {
     final keys = key.split(delimiter);
     dynamic current = result;

test/unflatten_test.dart

@@ -57,7 +57,7 @@ void main() {
       expect(result, expected);
     });
 
-    test('Fails on multiple Keys', () {
+    test('Fails on repeated key', () {
       const obj = {
         'hello.lorem.ipsum': 'again',
         'hello.lorem.dolor': 'sit',
@@ -69,6 +69,24 @@ void main() {
       expect(() => unflatten(obj), throwsArgumentError);
     });
 
+    test('Fails on nested Map', () {
+      const obj = {
+        'world': {'greet': 'hello'},
+      };
+
+      expect(() => unflatten(obj), throwsArgumentError);
+    });
+
+    test('Fails on nested nested Map', () {
+      const obj = {
+        'hello.lorem.ipsum': {
+          'world': {'greet': 'hello'},
+        },
+      };
+
+      expect(() => unflatten(obj), throwsArgumentError);
+    });
+
     test('Custom Delimiter', () {
       const obj = {
         'hello world again': 'good morning',