diff --git a/src/calibre/ebooks/rtf2xml/tokenize.py b/src/calibre/ebooks/rtf2xml/tokenize.py
index da4c1317e7..d19f3ef61f 100644
--- a/src/calibre/ebooks/rtf2xml/tokenize.py
+++ b/src/calibre/ebooks/rtf2xml/tokenize.py
@@ -62,7 +62,8 @@ class Tokenize:
             return token
         # change scope out
         elif token == r'\}':
-            self.__uc_value.pop()
+            if self.__uc_value:
+                self.__uc_value.pop()
             self.__reini_utf8_counters()
             return token
         # add a uc control