diff --git a/src/tagged-union.test.ts b/src/tagged-union.test.ts
index 42bf03b..e3e1d26 100644
--- a/src/tagged-union.test.ts
+++ b/src/tagged-union.test.ts
@@ -184,6 +184,9 @@ test("getTaggedUnionMatcher: a fallback receives unhandled boolean and nullish m
             | { readonly kind: false; readonly c: number }
             | { readonly kind: null; readonly d: number }
         >();
+        // `true`/`false`/`null` are keyed as `"true"`/`"false"`/`"null"`,
+        // but the fallback still receives the boolean/null, not the string.
+        assert.ok(s.kind === true || s.kind === false || s.kind === null);
         return 2 as const;
     });