diff --git a/lib/carto/tree/reference.js b/lib/carto/tree/reference.js
index 49914f1..a5ded89 100644
--- a/lib/carto/tree/reference.js
+++ b/lib/carto/tree/reference.js
@@ -1,3 +1,9 @@
+/*
+ * Carto pulls in a reference from the `mapnik-reference`
+ * module. This file builds indexes from that file for its various
+ * options, and provides validation methods for property: value
+ * combinations.
+ */
 (function(tree) {
 
 var _ = require('underscore');
@@ -101,31 +107,32 @@ tree.Reference.isFont = function(selector) {
 
 tree.Reference.validValue = function(env, selector, value) {
     var i, j;
-    if (value[0]) {
-        return tree.Reference.selector(selector).type == value[0].is;
-    } else {
-        // TODO: handle in reusable way
-        if (!tree.Reference.selector(selector)) {
-            return false;
-        } else if (value.value[0].is == 'keyword') {
-            return tree.Reference
-                .selector(selector).type
-                .indexOf(value.value[0].value) !== -1;
-        } else if (value.value[0].is == 'undefined') {
-            // caught earlier in the chain - ignore here so that
-            // error is not overridden
-            return true;
-        } else if (tree.Reference.selector(selector).type == 'numbers') {
-            for (i in value.value) {
-                if (value.value[i].is !== 'float') {
-                    return false;
-                }
+    // TODO: handle in reusable way
+    if (!tree.Reference.selector(selector)) {
+        return false;
+    } else if (value.value[0].is == 'keyword') {
+        return tree.Reference
+            .selector(selector).type
+            .indexOf(value.value[0].value) !== -1;
+    } else if (value.value[0].is == 'undefined') {
+        // caught earlier in the chain - ignore here so that
+        // error is not overridden
+        return true;
+    } else if (tree.Reference.selector(selector).type == 'numbers') {
+        for (i in value.value) {
+            if (value.value[i].is !== 'float') {
+                return false;
             }
+        }
+        return true;
+    } else if (tree.Reference.selector(selector).type == 'functions') {
+        // For backwards compatibility, you can specify a string for `functions`-compatible
+        // values, though they will not be validated.
+        if (value.value[0].is === 'string') {
             return true;
-        } else if (tree.Reference.selector(selector).type == 'functions') {
+        } else {
             for (i in value.value) {
                 for (j in value.value[i].value) {
-                    console.log(value.value[i]);
                     if (value.value[i].value[j].is !== 'call') {
                         return false;
                     }
@@ -140,22 +147,22 @@ tree.Reference.validValue = function(env, selector, value) {
                 }
             }
             return true;
-        } else {
-            if (tree.Reference.selector(selector).validate) {
-                var valid = false;
-                for (i = 0; i < value.value.length; i++) {
-                    if (tree.Reference.selector(selector).type == value.value[i].is &&
-                        tree.Reference
-                            ._validateValue
-                                [tree.Reference.selector(selector).validate]
-                                (env, value.value[i].value)) {
-                        return true;
-                    }
+        }
+    } else {
+        if (tree.Reference.selector(selector).validate) {
+            var valid = false;
+            for (i = 0; i < value.value.length; i++) {
+                if (tree.Reference.selector(selector).type == value.value[i].is &&
+                    tree.Reference
+                        ._validateValue
+                            [tree.Reference.selector(selector).validate]
+                            (env, value.value[i].value)) {
+                    return true;
                 }
-                return valid;
-            } else {
-                return tree.Reference.selector(selector).type == value.value[0].is;
             }
+            return valid;
+        } else {
+            return tree.Reference.selector(selector).type == value.value[0].is;
         }
     }
 };