diff --git a/tests/NetVips.Tests/IoFuncsTests.cs b/tests/NetVips.Tests/IoFuncsTests.cs
index 000c954..6222a4d 100644
--- a/tests/NetVips.Tests/IoFuncsTests.cs
+++ b/tests/NetVips.Tests/IoFuncsTests.cs
@@ -252,6 +252,7 @@ void EvalCallback(Image image, VipsProgress progressStruct)
 
             var buf = im.DzsaveBuffer("image-pyramid", suffix: ".png");
             Assert.True(buf.Length > 0);
+            Assert.True(false);
         }
 
         [Fact]