diff --git a/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/nullable-types.ts b/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/nullable-types.ts
index 5dcf874..56de267 100644
--- a/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/nullable-types.ts
+++ b/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/nullable-types.ts
@@ -1,7 +1,7 @@
 
 export type SimpleNullableRootType = {
-    long?: null | string;
-    uLong?: null | string;
+    long?: null | number;
+    uLong?: null | number;
     int?: null | number;
     uInt?: null | number;
     short?: null | number;
diff --git a/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/simple-types.ts b/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/simple-types.ts
index a96c587..6777eb3 100644
--- a/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/simple-types.ts
+++ b/TypeScript.ContractGenerator.Tests/Files/CustomGenerator/simple-types.ts
@@ -1,7 +1,7 @@
 
 export type SimpleRootType = {
-    long: string;
-    uLong: string;
+    long: number;
+    uLong: number;
     int: number;
     uInt: number;
     short: number;
diff --git a/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/nullable-types.ts b/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/nullable-types.ts
index 410f15b..d3da815 100644
--- a/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/nullable-types.ts
+++ b/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/nullable-types.ts
@@ -1,7 +1,7 @@
 
 export type SimpleNullableRootType = {
-    long?: null | string;
-    uLong?: null | string;
+    long?: null | number;
+    uLong?: null | number;
     int?: null | number;
     uInt?: null | number;
     short?: null | number;
diff --git a/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/simple-types.ts b/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/simple-types.ts
index 3adfedb..8cae4e1 100644
--- a/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/simple-types.ts
+++ b/TypeScript.ContractGenerator.Tests/Files/SimpleGenerator/simple-types.ts
@@ -1,7 +1,7 @@
 
 export type SimpleRootType = {
-    long: string;
-    uLong: string;
+    long: number;
+    uLong: number;
     int: number;
     uInt: number;
     short: number;
diff --git a/TypeScript.ContractGenerator/TypeBuilders/BuiltinTypeBuildingContext.cs b/TypeScript.ContractGenerator/TypeBuilders/BuiltinTypeBuildingContext.cs
index 899fe30..7052377 100644
--- a/TypeScript.ContractGenerator/TypeBuilders/BuiltinTypeBuildingContext.cs
+++ b/TypeScript.ContractGenerator/TypeBuilders/BuiltinTypeBuildingContext.cs
@@ -21,9 +21,9 @@ public static bool Accept(ITypeInfo type)
 
         protected override TypeScriptType ReferenceFromInternal(ITypeInfo type, TypeScriptUnit targetUnit, ITypeGenerator typeGenerator)
         {
-            if (builtinTypes.ContainsKey(type))
-                return new TypeScriptBuildInType(builtinTypes[type]);
-            throw new ArgumentOutOfRangeException();
+            if (builtinTypes.TryGetValue(type, out var typeScriptType))
+                return new TypeScriptBuildInType(typeScriptType);
+            throw new ArgumentOutOfRangeException(nameof(type), $"Type '{type}' is not found");
         }
 
         private static readonly Dictionary<ITypeInfo, string> builtinTypes = new Dictionary<ITypeInfo, string>
@@ -41,8 +41,8 @@ protected override TypeScriptType ReferenceFromInternal(ITypeInfo type, TypeScri
                 {TypeInfo.From<DateTime>(), "(Date | string)"},
                 {TypeInfo.From<TimeSpan>(), "(number | string)"},
                 {TypeInfo.From<string>(), "string"},
-                {TypeInfo.From<long>(), "string"},
-                {TypeInfo.From<ulong>(), "string"},
+                {TypeInfo.From<long>(), "number"},
+                {TypeInfo.From<ulong>(), "number"},
                 {TypeInfo.From<byte[]>(), "string"},
                 {TypeInfo.From<Guid>(), "string"},
                 {TypeInfo.From<char>(), "string"},
diff --git a/global.json b/global.json
index d58cbe0..b86006c 100644
--- a/global.json
+++ b/global.json
@@ -1,5 +1,6 @@
 {
   "sdk": {
-    "version": "5.0.302"
+    "version": "5.0.302",
+    "rollForward": "latestFeature"
   }
 }
\ No newline at end of file