diff --git a/lib/vows.js b/lib/vows.js
index 19ed4c1..77f08df 100644
--- a/lib/vows.js
+++ b/lib/vows.js
@@ -289,6 +289,8 @@ function addVows(tests) {
                         process.nextTick(function (val) {
                             return function () { ctx.emitter.emit("success", val) };
                         }(topic));
+                    } else if (typeof(topic) !== "undefined") {
+                        throw new(Error)("topic must not return anything when using `this.callback`.");
                     }
                     topic = ctx.emitter;
                 }