diff --git a/lib/dataUrls.js b/lib/dataUrls.js
index 5f5dc7e..5a170ad 100644
--- a/lib/dataUrls.js
+++ b/lib/dataUrls.js
@@ -47,7 +47,10 @@
 	scope.init = function(){
 		function listener(details){
 			const headers = details.responseHeaders;
-			if (settings.get("blockDataURLs", new URL(details.url))){
+			if (
+				details.statusCode !== 304 &&
+				settings.get("blockDataURLs", new URL(details.url))
+			){
 				const cspMatch = (blockBlob? "": "blob: ") + "filesystem: *";
 				logging.verbose("Adding CSP header to", details);
 				setHeader(headers, {