diff --git a/lib/carto/tree/filter.js b/lib/carto/tree/filter.js
index 3477b76..87c84f0 100644
--- a/lib/carto/tree/filter.js
+++ b/lib/carto/tree/filter.js
@@ -19,6 +19,10 @@ tree.Filter = function Filter(key, op, val, index, filename) {
         this.val = val;
     }
 
+    if (this.op !== '=' && this.op !== '!=') {
+        this.val = 1 * this.val;
+    }
+
     this.id = this.key + this.op + this.val;
 };
 
@@ -34,15 +38,12 @@ var opXML = {
 };
 
 tree.Filter.prototype.toXML = function(env) {
-    if (this.op !== '=' && this.op !== '!=') {
-        this.val = 1 * this.val;
-        if (isNaN(this.val)) {
-            env.error({
-                message: 'Cannot use operator "' + this.op + '" with value ' + this.val,
-                index: this.index,
-                filename: this.filename
-            });
-        }
+    if (this.op !== '=' && this.op !== '!=' && isNaN(this.val)) {
+        env.error({
+            message: 'Cannot use operator "' + this.op + '" with value ' + this.val,
+            index: this.index,
+            filename: this.filename
+        });
     }
     if (this.val.eval) this._val = this.val.eval(env);
     if (this.key.eval) this._key = this.key.eval(env);
diff --git a/test/errorhandling.test.js b/test/errorhandling.test.js
index e315038..cf2c118 100644
--- a/test/errorhandling.test.js
+++ b/test/errorhandling.test.js
@@ -23,7 +23,7 @@ helper.files('errorhandling', 'mml', function(file) {
             // @TODO for some reason, fs.readFile includes an additional \n
             // at the end of read files. Determine why.
             fs.readFile(helper.resultFile(file), 'utf8', function(err, data) {
-                assert.deepEqual(output, data.substr(0, data.length - 1));
+                if (!err) assert.deepEqual(output, data.substr(0, data.length - 1));
             });
         });
 
diff --git a/test/errorhandling/multi_stylesheets.mml b/test/errorhandling/multi_stylesheets.mml
new file mode 100644
index 0000000..27de05c
--- /dev/null
+++ b/test/errorhandling/multi_stylesheets.mml
@@ -0,0 +1,16 @@
+{
+    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+    "Stylesheet": [
+        "multi_stylesheets_b.mss",
+        "multi_stylesheets_a.mss"
+    ],
+    "Layer": [{
+        "id": "world",
+        "name": "world",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_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/errorhandling/multi_stylesheets_a.mss b/test/errorhandling/multi_stylesheets_a.mss
new file mode 100644
index 0000000..2db5417
--- /dev/null
+++ b/test/errorhandling/multi_stylesheets_a.mss
@@ -0,0 +1,6 @@
+#world {
+  polygon-opacity: 0.5;
+  line-width:4;
+  line-color:#fff;
+  line-opacity:0.3;
+}
diff --git a/test/errorhandling/multi_stylesheets_b.mss b/test/errorhandling/multi_stylesheets_b.mss
new file mode 100644
index 0000000..0c58439
--- /dev/null
+++ b/test/errorhandling/multi_stylesheets_b.mss
@@ -0,0 +1,3 @@
+#world {
+  polygonopacity: 0.5;
+}
diff --git a/test/rendering/nominzoom.mml b/test/rendering/nominzoom.mml
new file mode 100644
index 0000000..3edb411
--- /dev/null
+++ b/test/rendering/nominzoom.mml
@@ -0,0 +1,14 @@
+{
+    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+    "Stylesheet": [
+        "nominzoom.mss"
+    ],
+    "Layer": [{
+        "name": "world",
+        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_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/nominzoom.mss b/test/rendering/nominzoom.mss
new file mode 100644
index 0000000..af9c4b4
--- /dev/null
+++ b/test/rendering/nominzoom.mss
@@ -0,0 +1,3 @@
+#world[zoom < 5] {
+  polygon-fill: #000;
+}
diff --git a/test/rendering/nominzoom.result b/test/rendering/nominzoom.result
new file mode 100644
index 0000000..c2d3bd2
--- /dev/null
+++ b/test/rendering/nominzoom.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 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+
+
+<Style name="world" filter-mode="first">
+  <Rule>
+    <MinScaleDenominator>25000000</MinScaleDenominator>
+    <PolygonSymbolizer fill="#000000" />
+  </Rule>
+</Style>
+<Layer name="world"
+  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+    <StyleName>world</StyleName>
+    <Datasource>
+       <Parameter name="file"><![CDATA[[absolute path]]]></Parameter>
+       <Parameter name="type"><![CDATA[shape]]></Parameter>
+    </Datasource>
+  </Layer>
+
+</Map>