diff --git a/src/System.Text.Json/tests/System.Text.Json.Tests.csproj b/src/System.Text.Json/tests/System.Text.Json.Tests.csproj
index b00594b04bdb..5207830b29dc 100644
--- a/src/System.Text.Json/tests/System.Text.Json.Tests.csproj
+++ b/src/System.Text.Json/tests/System.Text.Json.Tests.csproj
@@ -2,7 +2,7 @@
   <PropertyGroup>
     <ProjectGuid>{5F553243-042C-45C0-8E49-C739131E11C3}</ProjectGuid>
     <Configurations>netcoreapp-Debug;netcoreapp-Release;netfx-Debug;netfx-Release;uap-Windows_NT-Debug;uap-Windows_NT-Release</Configurations>
-    <DefineConstants>$(DefineConstants);BUILDING_INBOX_LIBRARY</DefineConstants>
+    <DefineConstants Condition="'$(TargetGroup)'!='netfx'">$(DefineConstants);BUILDING_INBOX_LIBRARY</DefineConstants>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonTestPath)\System\IO\WrappedMemoryStream.cs">