diff --git a/lib/web/fetch/formdata-parser.js b/lib/web/fetch/formdata-parser.js
index 26c5b762536..bd8657da293 100644
--- a/lib/web/fetch/formdata-parser.js
+++ b/lib/web/fetch/formdata-parser.js
@@ -291,7 +291,7 @@ function parseMultipartFormDataHeaders (input, position) {
         if (input.indexOf(filenameBuffer, position.position) === position.position) {
           // 1. Advance position so it points at the byte after the next 0x22 (") byte
           //    (the one in the sequence of bytes matched above).
-          position.position += 13
+          position.position += 12
 
           // 2. Set filename to the result of parsing a multipart/form-data name given
           //    input and position, if the result is not failure. Otherwise, return failure.