diff --git a/src/Zio.Tests/TestSearchPattern.cs b/src/Zio.Tests/TestSearchPattern.cs
index 1d82170..f53ffc4 100644
--- a/src/Zio.Tests/TestSearchPattern.cs
+++ b/src/Zio.Tests/TestSearchPattern.cs
@@ -107,7 +107,7 @@ public void TestExtensions()
             var newPath = path.ChangeExtension(".zip");
             Assert.Equal("/a/b/c/d.zip", newPath.FullName);
             Assert.Equal(new UPath("a/b/c/d.txt"), path.ToRelative());
-            path.AssertNotNull();
+            Assert.Equal(path, path.AssertAbsolute());
             Assert.Throws<ArgumentNullException>(() => new UPath().AssertNotNull());
             Assert.Throws<ArgumentException>(() => new UPath("not_absolute").AssertAbsolute());
         }
diff --git a/src/Zio/FileSystems/SubFileSystem.cs b/src/Zio/FileSystems/SubFileSystem.cs
index a0b102e..c1a08f1 100644
--- a/src/Zio/FileSystems/SubFileSystem.cs
+++ b/src/Zio/FileSystems/SubFileSystem.cs
@@ -24,8 +24,7 @@ public class SubFileSystem : ComposeFileSystem
     /// <exception cref="DirectoryNotFoundException">If the directory subPath does not exist in the delegate FileSystem</exception>
     public SubFileSystem(IFileSystem fileSystem, UPath subPath, bool owned = true) : base(fileSystem, owned)
     {
-        subPath.AssertAbsolute(nameof(subPath));
-        SubPath = subPath;
+        SubPath = subPath.AssertAbsolute(nameof(subPath));
         if (!fileSystem.DirectoryExists(SubPath))
         {
             throw NewDirectoryNotFoundException(SubPath);
diff --git a/src/Zio/UPathExtensions.cs b/src/Zio/UPathExtensions.cs
index cb40e26..ebffd8b 100644
--- a/src/Zio/UPathExtensions.cs
+++ b/src/Zio/UPathExtensions.cs
@@ -240,11 +240,13 @@ public static bool IsInDirectory(this UPath path, UPath directory, bool recursiv
     /// <param name="path">The path.</param>
     /// <param name="name">The name of a parameter to include n the <see cref="ArgumentNullException"/>.</param>
     /// <exception cref="System.ArgumentNullException">If the path was null using the parameter name from <paramref name="name"/></exception>
-    public static void AssertNotNull(this UPath path, string name = "path")
+    public static UPath AssertNotNull(this UPath path, string name = "path")
     {
         if (path.IsNull)
             Throw(name);
 
+        return path;
+
         static void Throw(string name) => throw new ArgumentNullException(name);
     }
 
@@ -254,11 +256,13 @@ public static void AssertNotNull(this UPath path, string name = "path")
     /// <param name="path">The path.</param>
     /// <param name="name">The name of a parameter to include n the <see cref="ArgumentNullException"/>.</param>
     /// <exception cref="System.ArgumentException">If the path is not absolute using the parameter name from <paramref name="name"/></exception>
-    public static void AssertAbsolute(this UPath path, string name = "path")
+    public static UPath AssertAbsolute(this UPath path, string name = "path")
     {
         if (!path.IsAbsolute)
             Throw(path, name);
 
+        return path;
+
         static void Throw(UPath path, string name) => throw new ArgumentException($"Path `{path}` must be absolute", name);
     }
 }