diff --git a/Semantics.Quantities/Semantics.Quantities.csproj b/Semantics.Quantities/Semantics.Quantities.csproj
index f4a7ce5..f5842ed 100644
--- a/Semantics.Quantities/Semantics.Quantities.csproj
+++ b/Semantics.Quantities/Semantics.Quantities.csproj
@@ -5,5 +5,8 @@
   <PropertyGroup>
     <TargetFrameworks>net9.0;net8.0;net7.0;</TargetFrameworks>
   </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Polyfill" VersionOverride="9.7.1" />
+  </ItemGroup>
 
 </Project>
diff --git a/Semantics.Strings/Semantics.Strings.csproj b/Semantics.Strings/Semantics.Strings.csproj
index e08e5c4..efad27b 100644
--- a/Semantics.Strings/Semantics.Strings.csproj
+++ b/Semantics.Strings/Semantics.Strings.csproj
@@ -9,5 +9,8 @@
     <!-- CA2249: Consider using String.Contains instead of String.IndexOf -->
     <!-- IDE0057: Substring can be simplified -->
   </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Polyfill" VersionOverride="9.7.1" />
+  </ItemGroup>
 
 </Project>