diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 740ca05..245a968 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -174,6 +174,10 @@ jobs:
         site: http://ctm.crouchingtigerhiddenfruitbat.org/pub/cygwin/circa/64bit/2021/01/22/181752
       continue-on-error: true
 
+    # The install step should have failed due to the duff signature.  Check
+    # that by trying to run Bash: if Cygwin were installed correctly, we'd run
+    # Cygwin Bash; if it isn't, we'll run one of the non-Cygwin Bash
+    # executables provided by the GitHub runner.
     - name: Check Cygwin isn't installed
       run: |
         if [[ "${OSTYPE}" = "cygwin" ]]; then