diff --git a/src/utils.js b/src/utils.js
index 4f661946c..6a00bf02e 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -80,8 +80,8 @@ var cloneCssStyles = function(svg, classes){
                     }
                 }
             }
-            catch(err) {
-                if(rule !== 'undefined'){
+            catch (err) {
+                if (typeof(rule) !== 'undefined') {
                     log.warn('Invalid CSS selector "' + rule.selectorText + '"', err);
                 }
             }
@@ -134,4 +134,4 @@ var cloneCssStyles = function(svg, classes){
     }
 };
 
-export {cloneCssStyles};
\ No newline at end of file
+export {cloneCssStyles};
diff --git a/src/utils.spec.js b/src/utils.spec.js
index 95cda44cd..a42771821 100644
--- a/src/utils.spec.js
+++ b/src/utils.spec.js
@@ -104,6 +104,20 @@ describe('when cloning CSS ', function () {
         document.styleSheets[styleSheetCount].title = 'mermaid-svg-internal-css';
     }
 
+    it('should handle errors thrown when accessing CSS rules', function() {
+        var svg = document.createElement('svg');
+        svg.setAttribute('id', 'mermaid-01');
+
+        // Firefox throws a SecurityError when trying to access cssRules
+        document.styleSheets[document.styleSheets.length++] = {
+          get cssRules() { throw new Error('SecurityError'); }
+        };
+
+        expect(function() {
+          utils.cloneCssStyles(svg, {});
+        }).not.toThrow();
+    });
+
     it('should handle an empty set of classes', function () {
         var svg = document.createElement('svg');
         svg.setAttribute('id', 'mermaid-01');