diff --git a/test/functional/tools/output_filter.xml b/test/functional/tools/output_filter.xml
index c065d6d4afd3..fb80d523820d 100644
--- a/test/functional/tools/output_filter.xml
+++ b/test/functional/tools/output_filter.xml
@@ -59,7 +59,7 @@ echo 'p2.reverse' > p2.reverse
                 </assert_contents>
             </output>
         </test>
-        <!-- tool runs with no outputs should fail -->
+        <!-- a test that does not produce any outputs violates the silent assumption that a tool run should have an output, hence expect_test_failure -->
         <test expect_num_outputs="0" expect_test_failure="true">
             <param name="produce_out_1" value="false" />
             <param name="filter_text_1" value="not_foo_or_bar" />