diff --git a/lib/css-base.js b/lib/css-base.js
index 59af87df..bde17d85 100644
--- a/lib/css-base.js
+++ b/lib/css-base.js
@@ -54,7 +54,7 @@ function cssWithMappingToString(item, useSourceMap) {
 		return content;
 	}
 
-	if (useSourceMap && typeof btoa === 'function') {
+	if (useSourceMap && typeof window.btoa === 'function') {
 		var sourceMapping = toComment(cssMapping);
 		var sourceURLs = cssMapping.sources.map(function (source) {
 			return '/*# sourceURL=' + cssMapping.sourceRoot + source + ' */'
@@ -69,7 +69,7 @@ function cssWithMappingToString(item, useSourceMap) {
 // Adapted from convert-source-map (MIT)
 function toComment(sourceMap) {
 	// eslint-disable-next-line no-undef
-	var base64 = btoa(unescape(encodeURIComponent(JSON.stringify(sourceMap))));
+	var base64 = window.btoa(unescape(encodeURIComponent(JSON.stringify(sourceMap))));
 	var data = 'sourceMappingURL=data:application/json;charset=utf-8;base64,' + base64;
 
 	return '/*# ' + data + ' */';