diff --git a/src/calibre/ebooks/epub/output.py b/src/calibre/ebooks/epub/output.py
index 952559a9e2..17f6eb9f46 100644
--- a/src/calibre/ebooks/epub/output.py
+++ b/src/calibre/ebooks/epub/output.py
@@ -241,9 +241,12 @@ class EPUBOutput(OutputFormatPlugin):
                 self.log.debug('Encrypting font:', uri)
                 with open(path, 'r+b') as f:
                     data = f.read(1024)
-                    f.seek(0)
-                    for i in range(1024):
-                        f.write(chr(ord(data[i]) ^ key[i%16]))
+                    if len(data) >= 1024:
+                        f.seek(0)
+                        for i in range(1024):
+                            f.write(chr(ord(data[i]) ^ key[i%16]))
+                    else:
+                        self.log.warn('Font', path, 'is invalid, ignoring')
                 if not isinstance(uri, unicode):
                     uri = uri.decode('utf-8')
                 fonts.append(u'''