diff --git a/src/libraries/Common/tests/AppleTestRunner/AppleTestRunner.cs b/src/libraries/Common/tests/AppleTestRunner/AppleTestRunner.cs
index 2af27adacbe6d0..ab1c0ab9b28191 100644
--- a/src/libraries/Common/tests/AppleTestRunner/AppleTestRunner.cs
+++ b/src/libraries/Common/tests/AppleTestRunner/AppleTestRunner.cs
@@ -21,10 +21,10 @@ public class SimpleTestRunner : iOSApplicationEntryPoint, IDevice
 {
     // to be wired once https://github.com/dotnet/xharness/pull/46 is merged
     [DllImport("__Internal")]
-    public extern static void mono_ios_append_output (string value);
+    public extern static void mono_ios_append_output ([MarshalAs(UnmanagedType.LPUTF8Str)] string value);
 
     [DllImport("__Internal")]
-    public extern static void mono_ios_set_summary (string value);
+    public extern static void mono_ios_set_summary ([MarshalAs(UnmanagedType.LPUTF8Str)] string value);
 
     private static List<string> s_testLibs = new List<string>();
     private static List<Assembly>? s_testAssemblies;