diff --git a/src/leiningen/run.clj b/src/leiningen/run.clj
index 359ae70d..cd74f61f 100644
--- a/src/leiningen/run.clj
+++ b/src/leiningen/run.clj
@@ -6,6 +6,10 @@
            (clojure.lang Reflector)))
 
 (defn- normalize-main [given]
+  (when-not (or (symbol? given)
+                (and (string? given) (symbol? (read-string given))))
+    (main/abort (str "Option -m requires a valid namespace argument, not "
+                     given ".")))
   (if (namespace (symbol given))
     (symbol given)
     (symbol (name given) "-main")))
@@ -70,7 +74,11 @@
 
         ;; If we got an exception earlier and nothing else worked,
         ;; rethrow that.
-        (= :threw ns-flag#) (throw data#)))))
+        (= :threw ns-flag#)
+        (do (binding [*out* *err*]
+              (println (str "Can't find '" '~given "' as .class or .clj for "
+                            "lein run: please check the spelling.")))
+            (throw data#))))))
 
 (defn- run-main
   "Loads the project namespaces as well as all its dependencies and then calls
diff --git a/test/leiningen/test/run.clj b/test/leiningen/test/run.clj
index 05651332..168f045e 100644
--- a/test/leiningen/test/run.clj
+++ b/test/leiningen/test/run.clj
@@ -22,6 +22,17 @@
   (run tricky-name-project "-m" "org.domain.tricky-name.munch" "/unreadable")
   (is (= ":munched (\"/unreadable\")" (slurp out-file))))
 
+(deftest test-valid-namespace-argument
+  (is (re-find #"Option -m requires a valid namespace argument, not -1\."
+               (helper/abort-msg run tricky-name-project "-m" "-1"))))
+
+(deftest test-nonexistant-ns-error-message
+  (is (re-find #"Can't find 'nonexistant.ns' as \.class or \.clj for lein run"
+               (with-out-str
+                 (binding [*err* *out*]
+                   (try (run tricky-name-project "-m" "nonexistant.ns")
+                        (catch Exception _)))))))
+
 (deftest test-escape-args
   (run tricky-name-project "--" ":bbb")
   (is (= "nom::bbb" (slurp out-file)))