diff --git a/src/runtime/runtime.cs b/src/runtime/runtime.cs
index a1f9a38aa..c696af1b0 100644
--- a/src/runtime/runtime.cs
+++ b/src/runtime/runtime.cs
@@ -754,7 +754,7 @@ public static extern int Py_Main(
         internal static extern int PyRun_SimpleString(string code);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
-        internal static extern IntPtr PyRun_String(string code, IntPtr st, IntPtr globals, IntPtr locals);
+        internal static extern IntPtr PyRun_String([MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(Utf8Marshaler))] string code, IntPtr st, IntPtr globals, IntPtr locals);
 
         [DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
         internal static extern IntPtr PyEval_EvalCode(IntPtr co, IntPtr globals, IntPtr locals);