diff --git a/lib/fetch/dataURL.js b/lib/fetch/dataURL.js
index 04b5c39af23..e5d4ca1fa4a 100644
--- a/lib/fetch/dataURL.js
+++ b/lib/fetch/dataURL.js
@@ -418,8 +418,11 @@ function forgivingBase64 (data) {
   if (dataLength % 4 === 0) {
     // 1. If data ends with one or two U+003D (=) code points,
     // then remove them from data.
-    if (data.charCodeAt(dataLength - 1) === 0x003D && data.charCodeAt(--dataLength) === 0x003D) {
+    if (data.charCodeAt(dataLength - 1) === 0x003D) {
       --dataLength
+      if (data.charCodeAt(dataLength - 1) === 0x003D) {
+        --dataLength
+      }
     }
   }