diff --git a/src/NetVips/Image.cs b/src/NetVips/Image.cs
index 2426680..2bf58e2 100644
--- a/src/NetVips/Image.cs
+++ b/src/NetVips/Image.cs
@@ -30,7 +30,7 @@ public partial class Image : VipsObject
         /// Internal marshaller delegate for <see cref="EvalDelegate"/>.
         /// </summary>
         [SuppressUnmanagedCodeSecurity, UnmanagedFunctionPointer(CallingConvention.Cdecl)]
-        internal delegate void EvalMarshalDelegate(IntPtr imagePtr, VipsProgress progress, IntPtr userDataPtr);
+        internal delegate void EvalMarshalDelegate(IntPtr imagePtr, IntPtr progressPtr, IntPtr userDataPtr);
 
         /// <inheritdoc cref="VipsObject"/>
         internal Image(IntPtr pointer)
@@ -2103,9 +2103,9 @@ public void SetKill(bool kill)
         /// <exception cref="T:System.ArgumentException">If it failed to connect the signal.</exception>
         public ulong SignalConnect(Enums.Signals signal, EvalDelegate callback, IntPtr data = default)
         {
-            void EvalMarshal(IntPtr imagePtr, VipsProgress progress, IntPtr userDataPtr)
+            void EvalMarshal(IntPtr imagePtr, IntPtr progressPtr, IntPtr userDataPtr)
             {
-                if (imagePtr == IntPtr.Zero)
+                if (imagePtr == IntPtr.Zero || progressPtr == IntPtr.Zero)
                 {
                     return;
                 }
@@ -2113,6 +2113,8 @@ void EvalMarshal(IntPtr imagePtr, VipsProgress progress, IntPtr userDataPtr)
                 using var image = new Image(imagePtr);
                 image.ObjectRef();
 
+                var progress = Marshal.PtrToStructure<VipsProgress>(progressPtr);
+                
                 callback.Invoke(image, progress);
             }
 
diff --git a/src/NetVips/Internal/Vips.cs b/src/NetVips/Internal/Vips.cs
index ecd355a..4f28f4a 100644
--- a/src/NetVips/Internal/Vips.cs
+++ b/src/NetVips/Internal/Vips.cs
@@ -17,8 +17,8 @@ internal static class Vips
         internal delegate IntPtr TypeMap2Fn(IntPtr type, IntPtr a, IntPtr b);
 
         [SuppressUnmanagedCodeSecurity, UnmanagedFunctionPointer(CallingConvention.Cdecl)]
-        internal delegate IntPtr ArgumentMapFn(IntPtr @object, GParamSpec.Struct pspec, VipsArgumentClass argumentClass,
-            VipsArgumentInstance argumentInstance, IntPtr a, IntPtr b);
+        internal delegate IntPtr ArgumentMapFn(IntPtr @object, IntPtr pspec, IntPtr argumentClass,
+            IntPtr argumentInstance, IntPtr a, IntPtr b);
 
         [SuppressUnmanagedCodeSecurity, UnmanagedFunctionPointer(CallingConvention.Cdecl)]
         internal delegate int CallbackFn(IntPtr a, IntPtr b);
diff --git a/src/NetVips/Introspect.cs b/src/NetVips/Introspect.cs
index e329af1..b7fd953 100644
--- a/src/NetVips/Introspect.cs
+++ b/src/NetVips/Introspect.cs
@@ -189,16 +189,16 @@ void AddArg(string name, Enums.ArgumentFlags flags)
             }
             else
             {
-                IntPtr AddConstruct(IntPtr self, GParamSpec.Struct pspec, VipsArgumentClass argumentClass,
-                    VipsArgumentInstance argumentInstance, IntPtr a, IntPtr b)
+                IntPtr AddConstruct(IntPtr self, IntPtr pspec, IntPtr argumentClass, IntPtr argumentInstance,
+                    IntPtr a, IntPtr b)
                 {
-                    var flags = argumentClass.Flags;
+                    var flags = Marshal.PtrToStructure<VipsArgumentClass>(argumentClass).Flags;
                     if ((flags & Enums.ArgumentFlags.CONSTRUCT) == 0)
                     {
                         return IntPtr.Zero;
                     }
 
-                    var name = Marshal.PtrToStringAnsi(pspec.Name);
+                    var name = Marshal.PtrToStringAnsi(Marshal.PtrToStructure<GParamSpec.Struct>(pspec).Name);
 
                     AddArg(name, flags);
 
diff --git a/src/NetVips/VipsProgress.cs b/src/NetVips/VipsProgress.cs
index 1346aaf..a086d22 100644
--- a/src/NetVips/VipsProgress.cs
+++ b/src/NetVips/VipsProgress.cs
@@ -22,7 +22,8 @@ public struct GTimer
         /// <summary>
         /// Is the timer currently active?
         /// </summary>
-        public int Active;
+        [MarshalAs(UnmanagedType.I1)]
+        public bool Active;
     }
 
     /// <summary>
@@ -65,6 +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