diff --git a/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.cs b/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.cs
index 12ce6a290..08e2ed04a 100644
--- a/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.cs
+++ b/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.cs
@@ -71,7 +71,7 @@ public virtual string TransformText()
 			if (value != null && this.typeToKeyAndJumpMap.TryGetValue(value.GetType().TypeHandle, out keyValuePair))
 			{
 				writer.WriteArrayHeader(2);
-				writer.WriteInt32(keyValuePair.Key);
+				writer.Write(keyValuePair.Key);
 				switch (keyValuePair.Value)
 				{
 ");
diff --git a/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.tt b/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.tt
index 67b78a537..359e165e7 100644
--- a/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.tt
+++ b/src/MessagePack.SourceGenerator/Transforms/UnionTemplate.tt
@@ -34,7 +34,7 @@ using MsgPack = global::MessagePack;
 			if (value != null && this.typeToKeyAndJumpMap.TryGetValue(value.GetType().TypeHandle, out keyValuePair))
 			{
 				writer.WriteArrayHeader(2);
-				writer.WriteInt32(keyValuePair.Key);
+				writer.Write(keyValuePair.Key);
 				switch (keyValuePair.Value)
 				{
 <# for(var i = 0; i < Info.SubTypes.Length; i++) { var item = Info.SubTypes[i]; #>
diff --git a/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(Namespace)/Formatters.MessagePack.GeneratedMessagePackResolver.MyTestNamespace.IMyTypeFormatter.g.cs b/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(Namespace)/Formatters.MessagePack.GeneratedMessagePackResolver.MyTestNamespace.IMyTypeFormatter.g.cs
index be91b6294..f6e75173c 100644
--- a/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(Namespace)/Formatters.MessagePack.GeneratedMessagePackResolver.MyTestNamespace.IMyTypeFormatter.g.cs
+++ b/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(Namespace)/Formatters.MessagePack.GeneratedMessagePackResolver.MyTestNamespace.IMyTypeFormatter.g.cs
@@ -32,7 +32,7 @@ public void Serialize(ref MsgPack::MessagePackWriter writer, global::MyTestNames
 			if (value != null && this.typeToKeyAndJumpMap.TryGetValue(value.GetType().TypeHandle, out keyValuePair))
 			{
 				writer.WriteArrayHeader(2);
-				writer.WriteInt32(keyValuePair.Key);
+				writer.Write(keyValuePair.Key);
 				switch (keyValuePair.Value)
 				{
 					case 0:
diff --git a/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(NestingClass)/Formatters.MessagePack.GeneratedMessagePackResolver.ContainingClass.IMyTypeFormatter.g.cs b/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(NestingClass)/Formatters.MessagePack.GeneratedMessagePackResolver.ContainingClass.IMyTypeFormatter.g.cs
index 63f0c831e..5fd854188 100644
--- a/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(NestingClass)/Formatters.MessagePack.GeneratedMessagePackResolver.ContainingClass.IMyTypeFormatter.g.cs
+++ b/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(NestingClass)/Formatters.MessagePack.GeneratedMessagePackResolver.ContainingClass.IMyTypeFormatter.g.cs
@@ -32,7 +32,7 @@ public void Serialize(ref MsgPack::MessagePackWriter writer, global::ContainingC
 			if (value != null && this.typeToKeyAndJumpMap.TryGetValue(value.GetType().TypeHandle, out keyValuePair))
 			{
 				writer.WriteArrayHeader(2);
-				writer.WriteInt32(keyValuePair.Key);
+				writer.Write(keyValuePair.Key);
 				switch (keyValuePair.Value)
 				{
 					case 0:
diff --git a/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(None)/Formatters.MessagePack.GeneratedMessagePackResolver.IMyTypeFormatter.g.cs b/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(None)/Formatters.MessagePack.GeneratedMessagePackResolver.IMyTypeFormatter.g.cs
index 7d39b6866..b849b5ca4 100644
--- a/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(None)/Formatters.MessagePack.GeneratedMessagePackResolver.IMyTypeFormatter.g.cs
+++ b/tests/MessagePack.SourceGenerator.Tests/Resources/UnionFormatter(None)/Formatters.MessagePack.GeneratedMessagePackResolver.IMyTypeFormatter.g.cs
@@ -31,7 +31,7 @@ public void Serialize(ref MsgPack::MessagePackWriter writer, global::IMyType val
 			if (value != null && this.typeToKeyAndJumpMap.TryGetValue(value.GetType().TypeHandle, out keyValuePair))
 			{
 				writer.WriteArrayHeader(2);
-				writer.WriteInt32(keyValuePair.Key);
+				writer.Write(keyValuePair.Key);
 				switch (keyValuePair.Value)
 				{
 					case 0: