diff --git a/src/NetVips/Internal/GObject.cs b/src/NetVips/Internal/GObject.cs
index 1c764726..f919d7a7 100644
--- a/src/NetVips/Internal/GObject.cs
+++ b/src/NetVips/Internal/GObject.cs
@@ -15,7 +15,7 @@ internal static class GObject
         [StructLayout(LayoutKind.Sequential)]
         internal struct Struct
         {
-            internal GType.Instance GTypeInstance;
+            internal IntPtr GTypeInstance;
 
             internal uint RefCount;
 
@@ -76,12 +76,6 @@ internal struct GEnumClass
 
     internal static class GType
     {
-        [StructLayout(LayoutKind.Sequential)]
-        internal struct Instance
-        {
-            internal IntPtr GClass;
-        }
-
         [SuppressUnmanagedCodeSecurity]
         [DllImport(Libraries.GObject, CallingConvention = CallingConvention.Cdecl,
             EntryPoint = "g_type_name")]
@@ -218,7 +212,7 @@ internal static class GParamSpec
         [StructLayout(LayoutKind.Sequential)]
         internal struct Struct
         {
-            internal GType.Instance GTypeInstance;
+            internal IntPtr GTypeInstance;
 
             internal IntPtr Name;
 
diff --git a/src/NetVips/Internal/Vips.cs b/src/NetVips/Internal/Vips.cs
index fe1572fe..ecd355ad 100644
--- a/src/NetVips/Internal/Vips.cs
+++ b/src/NetVips/Internal/Vips.cs
@@ -260,16 +260,10 @@ internal static extern int SetFromString(VipsObjectManaged @object,
         internal static extern IntPtr GetDescription(VipsObjectManaged @object);
     }
 
-    [StructLayout(LayoutKind.Sequential)]
-    internal struct VipsArgument
-    {
-        internal IntPtr Pspec;
-    }
-
     [StructLayout(LayoutKind.Sequential)]
     internal struct VipsArgumentClass
     {
-        internal VipsArgument Parent;
+        internal IntPtr Parent;
         internal IntPtr ObjectClass;
         internal ArgumentFlags Flags;
 
@@ -280,7 +274,7 @@ internal struct VipsArgumentClass
     [StructLayout(LayoutKind.Sequential)]
     internal struct VipsArgumentInstance
     {
-        internal VipsArgument Parent;
+        internal IntPtr Parent;
         internal IntPtr ArgumentClass;
         internal IntPtr Object;
 
diff --git a/src/NetVips/VipsProgress.cs b/src/NetVips/VipsProgress.cs
index b0b4a53d..a086d224 100644
--- a/src/NetVips/VipsProgress.cs
+++ b/src/NetVips/VipsProgress.cs
@@ -66,6 +66,11 @@ public struct VipsProgress
         /// <summary>
         /// Start time.
         /// </summary>
-        public GTimer Start;
+        private IntPtr StartPtr;
+
+        /// <summary>
+        /// Start time.
+        /// </summary>
+        public GTimer Start => Marshal.PtrToStructure<GTimer>(StartPtr);
     }
 }
\ No newline at end of file