diff --git a/tests/README.md b/tests/README.md
index a413e16..ebacae4 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -24,6 +24,7 @@ need not match the order in `$region_highlight`.
 Normally, tests fail if `$expected_region_highlight` and `$region_highlight`
 have different numbers of elements.  Tests may set `$expected_mismatch` to an
 explanation string (like `$todo`) to avoid this and skip the cardinality check.
+`$expected_mismatch` is set implicitly if the `$todo` component is present.
 
 **Note**: `$region_highlight` uses the same `"$i $j $style"` syntax but
 interprets the indexes differently.
diff --git a/tests/test-highlighting.zsh b/tests/test-highlighting.zsh
index 7b32dbc..a01df43 100755
--- a/tests/test-highlighting.zsh
+++ b/tests/test-highlighting.zsh
@@ -145,7 +145,10 @@ run_test_internal() {
     local -a expected_highlight_zone; expected_highlight_zone=( ${(z)expected_region_highlight[i]} )
     integer exp_start=$expected_highlight_zone[1] exp_end=$expected_highlight_zone[2]
     local todo=
-    (( $+expected_highlight_zone[4] )) && todo="# TODO $expected_highlight_zone[4]"
+    if (( $+expected_highlight_zone[4] )); then
+      todo="# TODO $expected_highlight_zone[4]"
+      : ${expected_mismatch:="cardinality check disabled whilst regular test points are expected to fail"}
+    fi
     if ! (( $+region_highlight[i] )); then
       print -r -- "not ok $i - unmatched expectation ($exp_start $exp_end $expected_highlight_zone[3])"
       continue