3 files changed, 27 insertions, 3 deletions
diff --git a/library/cpp/protobuf/json/proto2json_printer.cpp b/library/cpp/protobuf/json/proto2json_printer.cpp
index 409147dc06..6ec7800ad3 100644
--- a/library/cpp/protobuf/json/proto2json_printer.cpp
+++ b/library/cpp/protobuf/json/proto2json_printer.cpp
@@ -234,7 +234,8 @@ namespace NProtobufJson {
                                     << field.full_name() << ".";
             }
         }
-        shouldPrintField = shouldPrintField || GetConfig().MissingSingleKeyMode == TProto2JsonConfig::MissingKeyDefault;
+        shouldPrintField = shouldPrintField ||
+            (GetConfig().MissingSingleKeyMode == TProto2JsonConfig::MissingKeyDefault && !field.containing_oneof());
 
         if (shouldPrintField) {
             switch (field.cpp_type()) {
diff --git a/library/cpp/protobuf/json/ut/proto2json_ut.cpp b/library/cpp/protobuf/json/ut/proto2json_ut.cpp
index d4d6d374e9..3f154645a1 100644
--- a/library/cpp/protobuf/json/ut/proto2json_ut.cpp
+++ b/library/cpp/protobuf/json/ut/proto2json_ut.cpp
@@ -74,6 +74,22 @@ Y_UNIT_TEST_SUITE(TProto2JsonFlatTest) {
 #undef DEFINE_FIELD
     }
 
+    Y_UNIT_TEST(TestOneOfDefault) {
+        using namespace ::google::protobuf;
+        TFlatOneOfDefault proto;
+        NJson::TJsonValue json;
+        TProto2JsonConfig cfg;
+        cfg.SetMissingSingleKeyMode(TProto2JsonConfig::MissingKeyDefault);
+        UNIT_ASSERT_NO_EXCEPTION(Proto2Json(proto, json, cfg));
+
+        UNIT_ASSERT(!json.Has("ChoiceOne"));
+        UNIT_ASSERT(!json.Has("ChoiceTwo"));
+
+        proto.SetChoiceOne("one");
+        UNIT_ASSERT_NO_EXCEPTION(Proto2Json(proto, json, cfg));
+        UNIT_ASSERT_EQUAL("one", json["ChoiceOne"].GetStringRobust());
+    }
+
     Y_UNIT_TEST(TestNameGenerator) {
         TNameGeneratorType proto;
         proto.SetField(42);
diff --git a/library/cpp/protobuf/json/ut/test.proto b/library/cpp/protobuf/json/ut/test.proto
index 6bc1984373..8cd3db813a 100644
--- a/library/cpp/protobuf/json/ut/test.proto
+++ b/library/cpp/protobuf/json/ut/test.proto
@@ -122,6 +122,13 @@ message TFlatDefault {
     optional string UserID = 20 [default = "some_id"];
 };
 
+message TFlatOneOfDefault {
+    oneof Choice {
+        string ChoiceOne = 1 [default = "one"];
+        string ChoiceTwo = 2 [default = "two"];
+    }
+}
+
 message TCompositeOptional {
     optional TFlatOptional Part = 1;
 };
@@ -206,9 +213,9 @@ message TSingleTimestamp {
 }
 
 message TExtensionField {
-	extensions 100 to 199;
+    extensions 100 to 199;
 }
 
 extend TExtensionField {
   optional int32 bar = 123;
-}
-\ No newline at end of file
+}