diff --git a/src/Zio/FileSystems/PhysicalFileSystem.cs b/src/Zio/FileSystems/PhysicalFileSystem.cs
index 494fd75..8d4fb87 100644
--- a/src/Zio/FileSystems/PhysicalFileSystem.cs
+++ b/src/Zio/FileSystems/PhysicalFileSystem.cs
@@ -970,7 +970,10 @@ protected override string ConvertPathToInternalImpl(UPath path)
 
     private static bool HasWindowsVolumeLabel(string path)
     {
-        return path.IndexOf(":\\", StringComparison.Ordinal) == 1;
+        if (!IsOnWindows)
+            throw new NotSupportedException($"{nameof(HasWindowsVolumeLabel)} is only supported on Windows platforms.");
+
+        return path.Length >= 3 && path[1] == ':' && path[2] is '\\' or '/';
     }
 
     /// <inheritdoc />