diff --git a/build/closure-compiler/Externs.js b/build/closure-compiler/Externs.js
new file mode 100644
index 0000000000..3bb9464a14
--- /dev/null
+++ b/build/closure-compiler/Externs.js
@@ -0,0 +1,50 @@
+// ********************************************
+// This source file serves *ONLY* to avoid some compilation errors when the 
+//      compiler uses the flag:
+//          --jscomp_error undefinedVars
+//
+// In this source are declared all variables from other programs that use 
+//      OpenLayers. This avoids the error of undefined variable for these names.
+//
+// NOTE: The compiler does not include externs files like this in the 
+//      compilation result.
+// ********************************************
+
+// Used in lib/Firebug/firebug.js when gecko_dom
+    var frames;
+
+// Check the console when using Firebug Lite
+    var console;
+
+// Proj4js
+    var Proj4js = {Proj: function(){}};
+
+// Check JSON in lib/OpenLayers/Format/JSON.js
+    var JSON = {};
+
+// Google Maps
+    var GMap2;
+    var G_NORMAL_MAP;
+    var GEvent;
+    var GLatLngBounds = function(){};
+    var GSize = function(x, y){};
+    var GPoint = function(x, y){};
+    var GLatLng = function(lat, lon){};
+    
+// Multimap
+    var MultimapViewer = function(div){};
+    var MMLatLon = function(lat, lon){};
+    var MMPoint = function(x, y){};
+
+//VirtualEarth
+    var VEMap = function(name){};
+    var VEPixel = function(x, y){};
+    var VELatLong = function(lat, lon){};
+    var Msn = {VE:{}};
+
+// Yahoo
+    var YMap = function(div, type, size){};
+    var YGeoPoint = function(lat, lon){};
+    var YCoordPoint = function(x, y){};
+    var YSize = function(w, h){};
+
diff --git a/tools/closure.py b/tools/closure.py
index fb5e1a4781..1f788edb5d 100644
--- a/tools/closure.py
+++ b/tools/closure.py
@@ -14,7 +14,11 @@ def minimize(code):
     ntf2 = tempfile.NamedTemporaryFile()
     ntf2.close()
 
-    os.system("java -jar %s --js %s --js_output_file %s" % (path, ntf.name, ntf2.name))
+    os.system(("java -jar %s --js %s --js_output_file %s" +
+            " --externs closure-compiler/Externs.js" +
+            " --jscomp_warning checkVars" +
+            " --jscomp_error checkRegExp" +
+            " --jscomp_error undefinedVars") % (path, ntf.name, ntf2.name))
     data = open(ntf2.name).read()
     os.unlink(ntf.name) 
     os.unlink(ntf2.name)