diff --git a/observers.js b/observers.js
index ef6fc17..308b2b7 100644
--- a/observers.js
+++ b/observers.js
@@ -1699,6 +1699,11 @@ function autoCancelPrevious(emit) {
         else {
             cancelPrevious = emit.apply(this, arguments);
         }
+
+        if (typeof cancelPrevious !== "function" && cancelPrevious !== void 0) {
+            throw new Error("A handler method can only return a function or undefined");
+        }
+
         return function cancelObserver() {
             if (cancelPrevious) cancelPrevious();
             cancelPrevious = void 0;