diff --git a/src/clj/clojure/core.clj b/src/clj/clojure/core.clj
index 65b3bc2da3..b3f364c60c 100644
--- a/src/clj/clojure/core.clj
+++ b/src/clj/clojure/core.clj
@@ -191,10 +191,14 @@
  with-meta (fn with-meta [^clojure.lang.IObj x m]
              (. x (withMeta m))))
 
+(def ^{:private true :dynamic true}
+  assert-valid-fdecl (fn [fdecl]))
+
 (def
  ^{:private true}
  sigs
  (fn [fdecl]
+   (assert-valid-fdecl fdecl)
    (let [asig 
          (fn [fdecl]
            (let [arglist (first fdecl)
@@ -5652,3 +5656,9 @@
                         (keepi (inc idx) (rest s))
                         (cons x (keepi (inc idx) (rest s)))))))))]
        (keepi 0 coll))))
+
+(defn- ^{:dynamic true} assert-valid-fdecl
+  "A good fdecl looks like (([a] ...) ([a b] ...)) near the end of defn."
+  [fdecl]
+  (if-let [bad-args (seq (remove #(vector? %) (map first fdecl)))]
+    (throw (IllegalArgumentException. (str "Parameter declaration " (first bad-args) " should be a vector")))))
diff --git a/test/clojure/test_clojure/def.clj b/test/clojure/test_clojure/def.clj
new file mode 100644
index 0000000000..950f2a6876
--- /dev/null
+++ b/test/clojure/test_clojure/def.clj
@@ -0,0 +1,16 @@
+;   Copyright (c) Rich Hickey. All rights reserved.
+;   The use and distribution terms for this software are covered by the
+;   Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
+;   which can be found in the file epl-v10.html at the root of this distribution.
+;   By using this software in any fashion, you are agreeing to be bound by
+;   the terms of this license.
+;   You must not remove this notice, or any other, from this software.
+
+(ns clojure.test-clojure.def
+  (:use clojure.test clojure.test-clojure.helpers
+        clojure.test-clojure.protocols))
+
+(deftest defn-error-messages
+  (testing "bad arglist forms"
+    (is (fails-with-cause? IllegalArgumentException '#"Parameter declaration arg1 should be a vector"
+          (eval-in-temp-ns (defn foo (arg1 arg2)))))))
diff --git a/test/clojure/test_clojure/helpers.clj b/test/clojure/test_clojure/helpers.clj
index 42e8804735..777b5f64a7 100644
--- a/test/clojure/test_clojure/helpers.clj
+++ b/test/clojure/test_clojure/helpers.clj
@@ -25,3 +25,27 @@
      (eval
       '(do ~@forms))))
 
+(defn causes
+  [^Throwable throwable]
+  (loop [causes []
+         t throwable]
+    (if t (recur (conj causes t) (.getCause t)) causes)))
+
+;; this is how I wish clojure.test/thrown? worked...
+;; Does body throw expected exception, anywhere in the .getCause chain?
+(defmethod assert-expr 'fails-with-cause?
+  [msg [_ exception-class msg-re & body :as form]]
+  `(try
+   ~@body
+   (report {:type :fail, :message ~msg, :expected '~form, :actual nil})
+   (catch Throwable t#
+     (if (some (fn [cause#]
+                 (and
+                  (= ~exception-class (class cause#))
+                  (re-find ~msg-re (.getMessage cause#))))
+               (causes t#))
+       (report {:type :pass, :message ~msg,
+                :expected '~form, :actual t#})
+       (report {:type :fail, :message ~msg,
+                :expected '~form, :actual t#})))))
+
diff --git a/test/clojure/test_clojure/protocols.clj b/test/clojure/test_clojure/protocols.clj
index 52aa346ffd..b0f48453f9 100644
--- a/test/clojure/test_clojure/protocols.clj
+++ b/test/clojure/test_clojure/protocols.clj
@@ -10,33 +10,10 @@
 
 (ns clojure.test-clojure.protocols
   (:use clojure.test clojure.test-clojure.protocols.examples)
-  (:require [clojure.test-clojure.protocols.more-examples :as other])
+  (:require [clojure.test-clojure.protocols.more-examples :as other]
+            clojure.test-clojure.helpers)
   (:import [clojure.test_clojure.protocols.examples ExampleInterface]))
 
-(defn causes
-  [^Throwable throwable]
-  (loop [causes []
-         t throwable]
-    (if t (recur (conj causes t) (.getCause t)) causes)))
-
-;; this is how I wish clojure.test/thrown? worked...
-;; Does body throw expected exception, anywhere in the .getCause chain?
-(defmethod assert-expr 'fails-with-cause?
-  [msg [_ exception-class msg-re & body :as form]]
-  `(try
-   ~@body
-   (report {:type :fail, :message ~msg, :expected '~form, :actual nil})
-   (catch Throwable t#
-     (if (some (fn [cause#]
-                 (and
-                  (= ~exception-class (class cause#))
-                  (re-find ~msg-re (.getMessage cause#))))
-               (causes t#))
-       (report {:type :pass, :message ~msg,
-                :expected '~form, :actual t#})
-       (report {:type :fail, :message ~msg,
-                :expected '~form, :actual t#})))))
-
 ;; temporary hack until I decide how to cleanly reload protocol
 (defn reload-example-protocols
   []