diff --git a/lib/carto/parser.js b/lib/carto/parser.js
index 1994859..316442f 100644
--- a/lib/carto/parser.js
+++ b/lib/carto/parser.js
@@ -530,12 +530,18 @@ carto.Parser = function Parser(env) {
                         zooms.push(z);
                         conditions++;
                     } else if (f) {
-                        filters.add(f);
+                        var err = filters.add(f);
+                        if (err) {
+                            throw makeError({
+                                message: err,
+                                index: i - 1
+                            });
+                        }
                         conditions++;
                     } else if (attachment) {
                         throw makeError({
-                            message:'Encountered second attachment name.',
-                            index:i - 1
+                            message: 'Encountered second attachment name.',
+                            index: i - 1
                         });
                     } else {
                         attachment = a;
diff --git a/lib/carto/tree/filterset.js b/lib/carto/tree/filterset.js
index d1134ee..2bec009 100644
--- a/lib/carto/tree/filterset.js
+++ b/lib/carto/tree/filterset.js
@@ -155,9 +155,37 @@ tree.Filterset.prototype.addable = function(filter) {
     }
 };
 
+// Does the new filter constitute a conflict?
+tree.Filterset.prototype.conflict = function(filter) {
+    var key = filter.key.toString(),
+        value = filter.val.toString();
+
+    if (!isNaN(parseFloat(value))) value = parseFloat(value);
+
+    // if (a=b) && (a=c)
+    // if (a=b) && (a!=b)
+    // or (a!=b) && (a=b)
+    if ((filter.op === '=' && key + '=' in this.filters &&
+        value != this.filters[key + '='].val.toString()) ||
+        (filter.op === '!=' && key + '=' in this.filters &&
+        value == this.filters[key + '='].val.toString()) ||
+        (filter.op === '=' && key + '!=' in this.filters &&
+        value == this.filters[key + '!='].val.toString())) {
+        return filter.toString() + ' added to ' + this.toString() + ' produces an invalid filter';
+    }
+
+    return false;
+};
+
 // Only call this function for filters that have been cleared by .addable().
-tree.Filterset.prototype.add = function(filter) {
-    var key = filter.key.toString(), id, op = filter.op, numval;
+tree.Filterset.prototype.add = function(filter, env) {
+    var key = filter.key.toString(),
+        id,
+        op = filter.op,
+        conflict = this.conflict(filter),
+        numval;
+
+    if (conflict) return conflict;
 
     if (op === '=') {
         for (var i in this.filters) {
diff --git a/test/errorhandling/contradiction.mml b/test/errorhandling/contradiction.mml
new file mode 100644
index 0000000..d6b4416
--- /dev/null
+++ b/test/errorhandling/contradiction.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": [
+        "contradiction.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=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
+        "Datasource": {
+            "file": "http://tilemill-data.s3.amazonaws.com/district.geojson",
+            "type": "ogr",
+            "layer": "OGRGeoJSON"
+        }
+    }]
+}
diff --git a/test/errorhandling/contradiction.mss b/test/errorhandling/contradiction.mss
new file mode 100644
index 0000000..648bf1f
--- /dev/null
+++ b/test/errorhandling/contradiction.mss
@@ -0,0 +1,3 @@
+#world[FeatureCla!=""][FeatureCla=""] {
+    polygon-fill: #fff;
+}
diff --git a/test/errorhandling/contradiction.result b/test/errorhandling/contradiction.result
new file mode 100644
index 0000000..63c7724
--- /dev/null
+++ b/test/errorhandling/contradiction.result
@@ -0,0 +1 @@
+contradiction.mss:1:37 [[FeatureCla]=] added to [FeatureCla]!= produces an invalid filter
diff --git a/test/errorhandling/contradiction_2.mss b/test/errorhandling/contradiction_2.mss
new file mode 100644
index 0000000..2aaa907
--- /dev/null
+++ b/test/errorhandling/contradiction_2.mss
@@ -0,0 +1,3 @@
+#world[FeatureCla=""][FeatureCla!=""] {
+    polygon-fill: #fff;
+}
diff --git a/test/errorhandling/contradiction_2.result b/test/errorhandling/contradiction_2.result
new file mode 100644
index 0000000..99588ed
--- /dev/null
+++ b/test/errorhandling/contradiction_2.result
@@ -0,0 +1 @@
+contradiction_2.mss:1:37 [[FeatureCla]!=] added to [FeatureCla]= produces an invalid filter
diff --git a/test/rendering/partial_overrides.mss b/test/rendering/partial_overrides.mss
index ea4a852..7ae9847 100644
--- a/test/rendering/partial_overrides.mss
+++ b/test/rendering/partial_overrides.mss
@@ -18,10 +18,6 @@
   line-color: #f00;
 }
 
-#world.m_5[NAME="United States"][NAME="Canada"] {
-  line-color: #f00;
-}
-
 #world.m_6[NAME!="Canada"][zoom > 5] {
   line-opacity: .7;
 }
diff --git a/test/rendering/partial_overrides.result b/test/rendering/partial_overrides.result
index 07bec60..f3ac99b 100644
--- a/test/rendering/partial_overrides.result
+++ b/test/rendering/partial_overrides.result
@@ -1,9 +1,9 @@
 <?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">
+<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="world" filter-mode="first">
+<Style name="world" filter-mode="first" >
   <Rule>
     <MaxScaleDenominator>1500000</MaxScaleDenominator>
     <Filter>([NAME] = 'United States') and ([POP] &gt; 100000000)</Filter>
@@ -80,22 +80,6 @@
     <Filter>([NAME] != 'Canada')</Filter>
     <LineSymbolizer stroke-width="5" />
   </Rule>
-  <Rule>
-    <MaxScaleDenominator>1500000</MaxScaleDenominator>
-    <Filter>([NAME] = 'Canada')</Filter>
-    <LineSymbolizer stroke="#ff0000" stroke-opacity="0.4" stroke-width="3" />
-  </Rule>
-  <Rule>
-    <MaxScaleDenominator>100000000</MaxScaleDenominator>
-    <MinScaleDenominator>1500000</MinScaleDenominator>
-    <Filter>([NAME] = 'Canada')</Filter>
-    <LineSymbolizer stroke="#ff0000" stroke-opacity="0.2" stroke-width="3" />
-  </Rule>
-  <Rule>
-    <MinScaleDenominator>100000000</MinScaleDenominator>
-    <Filter>([NAME] = 'Canada')</Filter>
-    <LineSymbolizer stroke="#ff0000" stroke-width="3" />
-  </Rule>
   <Rule>
     <MaxScaleDenominator>1500000</MaxScaleDenominator>
     <LineSymbolizer stroke-opacity="0.4" stroke-width="3" />
@@ -110,13 +94,12 @@
     <LineSymbolizer stroke-width="3" />
   </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=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
+<Layer name="world"
+  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
     <StyleName>world</StyleName>
     <Datasource>
-       <Parameter name="file">[absolute path]</Parameter>
-       <Parameter name="type">shape</Parameter>
+       <Parameter name="file"><![CDATA[[absolute path]]]></Parameter>
+       <Parameter name="type"><![CDATA[shape]]></Parameter>
     </Datasource>
   </Layer>