diff --git a/src/calibre/utils/browser.py b/src/calibre/utils/browser.py
index 2519b0ee1f..4b95e85712 100644
--- a/src/calibre/utils/browser.py
+++ b/src/calibre/utils/browser.py
@@ -58,7 +58,7 @@ class Browser(B):
         q = header.lower()
         for i, (k, v) in enumerate(tuple(self.addheaders)):
             if k.lower() == q:
-                self.addheaders[i] = value
+                self.addheaders[i] = (header, value)
                 found = True
         if not found:
             self.addheaders.append((header, value))