diff --git a/protoc_plugin/CHANGELOG.md b/protoc_plugin/CHANGELOG.md
index e5fad1f2..4ee43218 100644
--- a/protoc_plugin/CHANGELOG.md
+++ b/protoc_plugin/CHANGELOG.md
@@ -1,3 +1,9 @@
+## 22.0.2-wip
+
+* Fix factory argument types for protobuf `Map` fields. ([#975])
+
+[#975]: https://github.com/google/protobuf.dart/issues/975
+
 ## 22.0.1
 
 * Bump `protobuf` constraint to `^4.0.0`
diff --git a/protoc_plugin/lib/src/message_generator.dart b/protoc_plugin/lib/src/message_generator.dart
index b3020c8f..f05d16de 100644
--- a/protoc_plugin/lib/src/message_generator.dart
+++ b/protoc_plugin/lib/src/message_generator.dart
@@ -446,6 +446,12 @@ class MessageGenerator extends ProtobufContainer {
         if (field.isRepeated && !field.isMapField) {
           out.println(
               '  ${field.baseType.getRepeatedDartTypeIterable(fileGen)}? ${field.memberNames!.fieldName},');
+        } else if (field.isMapField) {
+          final keyType = field.getDartMapKeyType();
+          final valueType = field.getDartMapValueType();
+          out.println(
+              '  $coreImportPrefix.Iterable<$coreImportPrefix.MapEntry<$keyType, $valueType>>? '
+              '${field.memberNames!.fieldName},');
         } else {
           out.println(
               '  ${field.getDartType()}? ${field.memberNames!.fieldName},');
@@ -459,9 +465,12 @@ class MessageGenerator extends ProtobufContainer {
         if (field.isDeprecated) {
           out.println('    // ignore: deprecated_member_use_from_same_package');
         }
-        if (field.isRepeated || field.isMapField) {
+        if (field.isRepeated && !field.isMapField) {
           out.println(
               '    \$result.${field.memberNames!.fieldName}.addAll(${field.memberNames!.fieldName});');
+        } else if (field.isMapField) {
+          out.println(
+              '    \$result.${field.memberNames!.fieldName}.addEntries(${field.memberNames!.fieldName});');
         } else {
           out.println(
               '    \$result.${field.memberNames!.fieldName} = ${field.memberNames!.fieldName};');
diff --git a/protoc_plugin/lib/src/protobuf_field.dart b/protoc_plugin/lib/src/protobuf_field.dart
index b4ca5d1d..7906cbec 100644
--- a/protoc_plugin/lib/src/protobuf_field.dart
+++ b/protoc_plugin/lib/src/protobuf_field.dart
@@ -140,18 +140,36 @@ class ProtobufField {
     // for example in package:protobuf/src/protobuf/mixins/well_known.dart.
   }
 
-  /// Returns the expression to use for the Dart type.
+  /// Returns the type to use for the Dart field type.
   String getDartType() {
     if (isMapField) {
-      final d = baseType.generator as MessageGenerator;
-      final keyType = d._fieldList[0].baseType.getDartType(parent.fileGen!);
-      final valueType = d._fieldList[1].baseType.getDartType(parent.fileGen!);
+      final keyType = getDartMapKeyType();
+      final valueType = getDartMapValueType();
       return '$protobufImportPrefix.PbMap<$keyType, $valueType>';
     }
     if (isRepeated) return baseType.getRepeatedDartType(parent.fileGen!);
     return baseType.getDartType(parent.fileGen!);
   }
 
+  /// Only for map fields: returns the type to use for Dart map field key type.
+  String getDartMapKeyType() {
+    assert(isMapField);
+    return (baseType.generator as MessageGenerator)
+        ._fieldList[0]
+        .baseType
+        .getDartType(parent.fileGen!);
+  }
+
+  /// Only for map fields: returns the type to use for Dart map field value
+  /// type.
+  String getDartMapValueType() {
+    assert(isMapField);
+    return (baseType.generator as MessageGenerator)
+        ._fieldList[1]
+        .baseType
+        .getDartType(parent.fileGen!);
+  }
+
   /// Returns the tag number of the underlying proto field.
   int get number => descriptor.number;
 
diff --git a/protoc_plugin/pubspec.yaml b/protoc_plugin/pubspec.yaml
index 9e58fd3a..95c33d3b 100644
--- a/protoc_plugin/pubspec.yaml
+++ b/protoc_plugin/pubspec.yaml
@@ -1,5 +1,5 @@
 name: protoc_plugin
-version: 22.0.1
+version: 22.0.2-wip
 description: A protobuf protoc compiler plugin used to generate Dart code.
 repository: https://github.com/google/protobuf.dart/tree/master/protoc_plugin
 
diff --git a/protoc_plugin/test/map_field_test.dart b/protoc_plugin/test/map_field_test.dart
index 038ec671..790023fd 100644
--- a/protoc_plugin/test/map_field_test.dart
+++ b/protoc_plugin/test/map_field_test.dart
@@ -7,6 +7,7 @@ import 'dart:convert';
 import 'package:protobuf/protobuf.dart';
 import 'package:test/test.dart';
 
+import '../out/protos/constructor_args/map_field.pb.dart' as with_constructors;
 import '../out/protos/map_field.pb.dart';
 
 void main() {
@@ -454,4 +455,18 @@ void main() {
       msg.int32ToInt32Field[0] = 1;
     }, throwsA(const TypeMatcher<UnsupportedError>()));
   });
+
+  test('Constructor map arguments accept key-value iterators', () {
+    final msg = with_constructors.TestMap(
+      stringToInt32Field: [
+        MapEntry('a', 1),
+        MapEntry('b', 2),
+        MapEntry('a', 3)
+      ],
+      int32ToStringField: {1: 'hi'}.entries,
+    );
+    expect(msg.stringToInt32Field['a'], 3);
+    expect(msg.stringToInt32Field['b'], 2);
+    expect(msg.int32ToStringField[1], 'hi');
+  });
 }