diff --git a/Makefile b/Makefile
index 16d07a1..60277de 100644
--- a/Makefile
+++ b/Makefile
@@ -3,7 +3,6 @@
 #
 
 expresso = ./node_modules/.bin/mocha
-docco = ./node_modules/.bin/docco
 
 lint:
 	./node_modules/.bin/jshint lib/carto/*.js lib/carto/tree/*.js
@@ -18,7 +17,4 @@ endif
 
 check: test
 
-doc:
-	$(docco) lib/carto/*.js lib/carto/tree/*.js
-
 .PHONY: test
diff --git a/lib/carto/parser.js b/lib/carto/parser.js
index 4679813..88a0c2f 100644
--- a/lib/carto/parser.js
+++ b/lib/carto/parser.js
@@ -662,10 +662,11 @@ carto.Parser = function Parser(env) {
                 save();
                 var key, op, val;
                 if (! $('[')) return;
-                if (key = $(/^[a-zA-Z0-9\-_]+/) || $(this.entities.quoted) || $(this.entities.variable)) {
+                if (key = $(/^[a-zA-Z0-9\-_]+/) || $(this.entities.quoted) || $(this.entities.variable) || $(this.entities.field)) {
                     if ((op = $(this.entities.comparison)) &&
-                        (val = $(this.entities.quoted) || $(this.entities.variable) || $(/^[\w\-\.]+/))) {
+                        (val = $(this.entities.quoted) || $(this.entities.variable) || $(this.entities.dimension))) {
                         if (! $(']')) return;
+                        if (!key.is) key = new tree.Field(key);
                         return new tree.Filter(key, op, val, memo, env.filename);
                     }
                 }
diff --git a/lib/carto/tree/filter.js b/lib/carto/tree/filter.js
index 29d3c1d..9dd33c4 100644
--- a/lib/carto/tree/filter.js
+++ b/lib/carto/tree/filter.js
@@ -1,28 +1,12 @@
 (function(tree) {
 
 tree.Filter = function Filter(key, op, val, index, filename) {
-    if (key.is) {
-        this.key = key.value;
-        this._key = key;
-    } else {
-        this.key = key;
-    }
-
+    this.key = key;
     this.op = op;
+    this.val = val;
     this.index = index;
     this.filename = filename;
 
-    if (val.is) {
-        this.val = val.value;
-        this._val = val;
-    } else {
-        this.val = val;
-    }
-
-    if (ops[this.op][1] == 'numeric') {
-        this.val = 1 * this.val;
-    }
-
     this.id = this.key + this.op + this.val;
 };
 
@@ -39,15 +23,14 @@ var ops = {
 };
 
 tree.Filter.prototype.toXML = function(env) {
-    var val, key;
-    if (this.val['eval']) this._val = this.val['eval'](env);
-    if (this.key['eval']) this._key = this.key['eval'](env);
-    if (this._key) key = this._key.toString(false);
-    if (this._val) val = this._val.toString(this._val.is == 'string');
+    var key = this.key['eval'](env);
+    var val = this.val['eval'](env);
+    key = key.toString(false);
+    val = val.toString(val.is == 'string');
 
     if (
-        (ops[this.op][1] == 'numeric' && isNaN(this.val)) ||
-        (ops[this.op][1] == 'string' && (val || this.val)[0] != "'")
+        (ops[this.op][1] == 'numeric' && isNaN(val)) ||
+        (ops[this.op][1] == 'string' && (val)[0] != "'")
     ) {
         env.error({
             message: 'Cannot use operator "' + this.op + '" with value ' + this.val,
@@ -56,7 +39,7 @@ tree.Filter.prototype.toXML = function(env) {
         });
     }
 
-    return '[' + (key || this.key) + ']' + ops[this.op][0] + '' + (val || this.val) + (ops[this.op][2] || '');
+    return key + ops[this.op][0] + '' + val + (ops[this.op][2] || '');
 };
 
 tree.Filter.prototype.toString = function() {
diff --git a/lib/carto/tree/variable.js b/lib/carto/tree/variable.js
index dc2ef4a..96da224 100644
--- a/lib/carto/tree/variable.js
+++ b/lib/carto/tree/variable.js
@@ -7,6 +7,10 @@ tree.Variable = function Variable(name, index, filename) {
 };
 
 tree.Variable.prototype = {
+    is: 'variable',
+    toString: function() {
+        return this.name;
+    },
     eval: function(env) {
         var variable,
             v,
diff --git a/package.json b/package.json
index bc78e76..28c2456 100644
--- a/package.json
+++ b/package.json
@@ -42,7 +42,6 @@
   },
   "devDependencies": {
     "mocha": "1.3.x",
-    "docco": "0.3.x",
     "jshint": "0.2.x",
     "sax": "0.1.x"
   },
diff --git a/test/rendering/support4504.mml b/test/rendering/support4504.mml
new file mode 100644
index 0000000..fd33922
--- /dev/null
+++ b/test/rendering/support4504.mml
@@ -0,0 +1,15 @@
+{
+    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+    "Stylesheet": [
+        "support4504.mss"
+    ],
+    "Layer": [{
+        "class": "new",
+        "name": "countries",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/test_data/shape_demo.zip",
+            "type": "shape"
+        }
+    }]
+}
diff --git a/test/rendering/support4504.mss b/test/rendering/support4504.mss
new file mode 100644
index 0000000..ef0f507
--- /dev/null
+++ b/test/rendering/support4504.mss
@@ -0,0 +1,7 @@
+@z13: 19;
+@num_pixels: 50;
+@z13area: @z13 * @z13 * @num_pixels;
+
+#countries[way_area > @z13area] {
+    polygon-opacity:0.2;
+}
diff --git a/test/rendering/support4504.result b/test/rendering/support4504.result
new file mode 100644
index 0000000..4cb4947
--- /dev/null
+++ b/test/rendering/support4504.result
@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE Map[]>
+<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over" maximum-extent="-20037508.34,-20037508.34,20037508.34,20037508.34">
+
+
+<Style name="countries" filter-mode="first" >
+  <Rule>
+    <Filter>([way_area] &gt; 18050)</Filter>
+    <PolygonSymbolizer fill-opacity="0.2" />
+  </Rule>
+</Style>
+<Layer name="countries"
+  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+    <StyleName>countries</StyleName>
+    <Datasource>
+       <Parameter name="file"><![CDATA[[absolute path]]]></Parameter>
+       <Parameter name="type"><![CDATA[shape]]></Parameter>
+    </Datasource>
+  </Layer>
+
+</Map>