diff --git a/src/alumbra/canonical/literal_value.clj b/src/alumbra/canonical/literal_value.clj
index bced96d..257f815 100644
--- a/src/alumbra/canonical/literal_value.clj
+++ b/src/alumbra/canonical/literal_value.clj
@@ -20,13 +20,13 @@
          (throw
            (IllegalArgumentException.
              (format
-               "null value not allowed for expected type '%s'."
+               "Null value not allowed for expected type '%s'."
                (type-shorthand t#)))))
        (when-not ~predicate
          (throw
            (IllegalArgumentException.
              (format
-               "value does not match expected type '%s': %s"
+               "Value does not match expected type '%s': %s"
                (type-shorthand t#)
                v#)))))))
 
diff --git a/src/alumbra/canonical/variables.clj b/src/alumbra/canonical/variables.clj
index 0a0fa67..2b0ebf8 100644
--- a/src/alumbra/canonical/variables.clj
+++ b/src/alumbra/canonical/variables.clj
@@ -9,19 +9,26 @@
    {:keys [alumbra/variable-name
            alumbra/default-value
            alumbra/type]}]
-  (let [v (get variables variable-name ::none)
-        type (as-type-description type)]
-    (if (= v ::none)
-      (if default-value
-        (resolve-value opts type default-value)
-        ::none)
-      (resolve-literal-value opts type v))))
+  (try
+    (let [v (get variables variable-name ::none)
+          type (as-type-description type)]
+      (if (= v ::none)
+        (if default-value
+          (resolve-value opts type default-value)
+          (resolve-literal-value opts type nil))
+        (resolve-literal-value opts type v)))
+    (catch IllegalArgumentException t
+      (throw
+        (IllegalArgumentException.
+          (format "Variable '$%s': %s"
+                  variable-name
+                  (.getMessage t))
+          t)))))
 
 (defn resolve-variables
   [opts {:keys [alumbra/variables]}]
   (->> (for [{:keys [alumbra/variable-name] :as variable} variables
-             :let [variable-value (resolve-variable opts variable)]
-             :when (not= variable-value ::none)]
+             :let [variable-value (resolve-variable opts variable)]]
          [variable-name variable-value])
        (into {})
        (assoc opts :variables)))
diff --git a/test/alumbra/canonicalizer_test.clj b/test/alumbra/canonicalizer_test.clj
index 39b4ca5..c279315 100644
--- a/test/alumbra/canonicalizer_test.clj
+++ b/test/alumbra/canonicalizer_test.clj
@@ -134,11 +134,11 @@
       (let [query "query ($q: CatQuery!) { randomCat(q: $q) { name } }"]
         (is (thrown-with-msg?
               IllegalArgumentException
-              #"null value not allowed"
+              #"Null value not allowed"
               (canonicalize query {})))
         (is (thrown-with-msg?
               IllegalArgumentException
-              #"null value not allowed"
+              #"Null value not allowed"
               (canonicalize query {"q" nil})))))))
 
 (deftest t-variable-type-mismatch
@@ -178,7 +178,7 @@
     (testing "nullability mismatch."
       (is (thrown-with-msg?
             IllegalArgumentException
-            #"null value not allowed for expected type 'CatQuery!'"
+            #"Null value not allowed for expected type 'CatQuery!'"
             (canonicalize
               "query ($q: CatQuery!) {
                randomCat(q: $q) { name }