diff --git a/src/outpace/config.clj b/src/outpace/config.clj
index 02ec7a5..c5e5070 100644
--- a/src/outpace/config.clj
+++ b/src/outpace/config.clj
@@ -175,13 +175,20 @@
   "Finds and loads config."
   []
   (if-let [source (find-config-source)]
-    (read-config source)
+    (with-meta (read-config source) :source source)
     {}))
 
 (def config
   "The delayed map of explicit configuration values."
   (delay (load-config)))
 
+(defn config-source
+  "Returns the source of config as string or nil if no source was
+  found."
+  []
+  (when-let [source (:source (meta @config))]
+    (str source)))
+
 (defn present?
   "Returns true if a configuration entry exists for the qname and, if an
    Optional value, the value is provided."
@@ -272,7 +279,10 @@
          (if (present? qname#)
            (alter-var-root var# (constantly (lookup qname#)))
            (when (and (-> var# meta :required) (not generating?))
-             (throw (Exception. (str "Missing required value for config var: " qname#)))))
+             (let [message# (if-let [source# (config-source)]
+                              (str "Config var " qname# " must define a default or be specified in " source#)
+                              (str "Config var " qname# " must define a default when there is no config source"))]
+               (throw (Exception. message#)))))
          (when-let [validate# (and (bound? var#) (-> var# meta :validate))]
            (validate @var# qname# validate#)))
        var#))
diff --git a/src/outpace/config/bootstrap.clj b/src/outpace/config/bootstrap.clj
index 87124bc..32464e9 100644
--- a/src/outpace/config/bootstrap.clj
+++ b/src/outpace/config/bootstrap.clj
@@ -22,4 +22,5 @@
         (io/resource res)
         (or (System/getProperty "config.edn")
             (when (.exists (io/file "config.edn"))
-              "config.edn")))))
+              "config.edn")))
+      (println "WARNING: no config source could be found.")))
diff --git a/test/outpace/config_test.clj b/test/outpace/config_test.clj
index 4a81cef..4802bee 100644
--- a/test/outpace/config_test.clj
+++ b/test/outpace/config_test.clj
@@ -444,3 +444,9 @@
     (is (not (c/provided? #{(extractable "bar" false)})))
     (is (not (c/provided? (list (extractable "bar" false)))))
     (is (not (c/provided? [(extractable "bar" false)])))))
+
+(deftest config-source-test
+  (let [source "config.clj"
+        config (delay (with-meta {} {:source source}))]
+    (with-redefs [c/config config]
+      (is (= source (c/config-source))))))