diff --git a/lib/pure/base64.nim b/lib/pure/base64.nim
index 3b8fb9b681..f238862508 100644
--- a/lib/pure/base64.nim
+++ b/lib/pure/base64.nim
@@ -255,6 +255,9 @@ proc decode*(s: string): string =
   while inputIndex <= inputEnds:
     while s[inputIndex] in {'\n', '\r', ' '}:
       inc inputIndex
+    # double check inputIndex as it can be incremented due to whitespace
+    if inputIndex > inputEnds:
+      break
     inputChar(a)
     inputChar(b)
     inputChar(c)