diff --git a/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs b/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
index c7153264ae..3d44fa988a 100644
--- a/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
+++ b/framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs
@@ -47,7 +47,7 @@ public static class AbpStringExtensions
     /// Indicates whether this string is null or an System.String.Empty string.
     /// </summary>
     [ContractAnnotation("str:null => true")]
-    public static bool IsNullOrEmpty(this string? str)
+    public static bool IsNullOrEmpty([System.Diagnostics.CodeAnalysis.NotNullWhen(false)]this string? str)
     {
         return string.IsNullOrEmpty(str);
     }
@@ -56,7 +56,7 @@ public static class AbpStringExtensions
     /// indicates whether this string is null, empty, or consists only of white-space characters.
     /// </summary>
     [ContractAnnotation("str:null => true")]
-    public static bool IsNullOrWhiteSpace(this string? str)
+    public static bool IsNullOrWhiteSpace([System.Diagnostics.CodeAnalysis.NotNullWhen(false)]this string? str)
     {
         return string.IsNullOrWhiteSpace(str);
     }
diff --git a/framework/src/Volo.Abp.Core/Volo.Abp.Core.csproj b/framework/src/Volo.Abp.Core/Volo.Abp.Core.csproj
index c250d07eee..9709051409 100644
--- a/framework/src/Volo.Abp.Core/Volo.Abp.Core.csproj
+++ b/framework/src/Volo.Abp.Core/Volo.Abp.Core.csproj
@@ -37,4 +37,10 @@
   <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETStandard' And $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '2.1')) ">
     <PackageReference Include="System.ComponentModel.Annotations" Version="5.0.0" />
   </ItemGroup>
+  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETStandard' And $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '2.0')) ">
+    <PackageReference Include="Nullable" Version="1.3.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
 </Project>