diff --git a/tests/NetVips.Tests/IoFuncsTests.cs b/tests/NetVips.Tests/IoFuncsTests.cs
index a7901f6..9c010c6 100644
--- a/tests/NetVips.Tests/IoFuncsTests.cs
+++ b/tests/NetVips.Tests/IoFuncsTests.cs
@@ -9,8 +9,11 @@ namespace NetVips.Tests
 
     public class IoFuncsTests : IClassFixture<TestsFixture>
     {
+        private readonly ITestOutputHelper _testOutputHelper;
+
         public IoFuncsTests(TestsFixture testsFixture, ITestOutputHelper output)
         {
+            _testOutputHelper = output;
             testsFixture.SetUpLogging(output);
         }
 
@@ -240,8 +243,9 @@ public void TestSetProgress()
             var progress = new Progress<int>(percent => lastPercent = percent);
             im.SetProgress(progress);
 
-            var buf = im.DzsaveBuffer("image-pyramid");
+            var buf = im.DzsaveBuffer("image-pyramid", suffix: ".png");
             Assert.True(buf.Length > 0);
+            _testOutputHelper.WriteLine(lastPercent.ToString());
             Assert.True(lastPercent <= 100);
         }