diff --git a/linkcheck/checker/fileurl.py b/linkcheck/checker/fileurl.py
index 88acbcf6..8efcbd3f 100644
--- a/linkcheck/checker/fileurl.py
+++ b/linkcheck/checker/fileurl.py
@@ -106,7 +106,8 @@ class FileUrl (urlbase.UrlBase):
         base_url = re.sub("^file://(/?)([a-zA-Z]):", r"file:///\2|", base_url)
         # norm base url again after changing
         if self.base_url != base_url:
-            self.base_url, is_idn = linkcheck.url.url_norm(base_url)
+            base_url, is_idn = linkcheck.url.url_norm(base_url)
+            self.base_url = unicode(base_url)
 
     def build_url (self):
         """