added if

lib/single-file/css-rules-matcher.js

@@ -111,13 +111,15 @@ this.RulesMatcher = this.RulesMatcher || (() => {
 				const specificity = computeSpecificity(selector);
 				specificity.ruleIndex = ruleIndex;
 				specificity.sheetIndex = sheetIndex;
-				let elementRuleIndex = 0;
-				while (elementRuleIndex < elementInfo.length && elementInfo[elementRuleIndex].cssRule != cssRule) {
-					elementRuleIndex++;
-				}
 				let ruleInfo;
-				if (elementRuleIndex < elementInfo.length && elementInfo[elementRuleIndex].cssRule == cssRule) {
-					ruleInfo = elementInfo[elementRuleIndex];
+				if (elementInfo.length) {
+					let elementRuleIndex = 0;
+					while (elementRuleIndex < elementInfo.length && elementInfo[elementRuleIndex].cssRule != cssRule) {
+						elementRuleIndex++;
+					}
+					if (elementRuleIndex < elementInfo.length && elementInfo[elementRuleIndex].cssRule == cssRule) {
+						ruleInfo = elementInfo[elementRuleIndex];
+					}
 				}
 				if (ruleInfo) {
 					if (compareSpecificity(ruleInfo.specificity, specificity) == 1) {