diff --git a/test/blackbox-tests/test-cases/github2990/run.t b/test/blackbox-tests/test-cases/github2990/run.t
index 63e4ebbb0bf..8156448c780 100644
--- a/test/blackbox-tests/test-cases/github2990/run.t
+++ b/test/blackbox-tests/test-cases/github2990/run.t
@@ -1,3 +1,9 @@
 Test for problem in #2990
 
   $ dune runtest
+
+Make sure that `output.actual` is not inferred as a target:
+
+  $ dune build output.actual
+  Error: Don't know how to build output.actual
+  [1]