diff --git a/Parser/Parser.csproj b/Parser/Parser.csproj
index 180cb57..0922761 100644
--- a/Parser/Parser.csproj
+++ b/Parser/Parser.csproj
@@ -2,7 +2,7 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <LangVersion>8.0</LangVersion>
-    <NullableReferenceTypes>true</NullableReferenceTypes>
+    <NullableContextOptions>enable</NullableContextOptions>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="System.Collections.Immutable" Version="1.5.0" />