diff --git a/tests/NetVips.Tests/IoFuncsTests.cs b/tests/NetVips.Tests/IoFuncsTests.cs
index e4d83c8..ed670e2 100644
--- a/tests/NetVips.Tests/IoFuncsTests.cs
+++ b/tests/NetVips.Tests/IoFuncsTests.cs
@@ -248,6 +248,8 @@ void EvalCallback(Image image, VipsProgress progressStruct)
                 _testOutputHelper.WriteLine($"   active = {progressStruct.Start.Active} (timer active)");
             }
 
+            _testOutputHelper.WriteLine($"sizeof(VipsProgress): {Marshal.SizeOf<VipsProgress>()}");
+
             im.SetProgress(true);
             im.SignalConnect(Enums.Signals.PreEval, EvalCallback);
             im.SignalConnect(Enums.Signals.Eval, EvalCallback);