diff --git a/src/calibre/utils/formatter.py b/src/calibre/utils/formatter.py
index 0711013437..eff38203a0 100644
--- a/src/calibre/utils/formatter.py
+++ b/src/calibre/utils/formatter.py
@@ -294,11 +294,11 @@ class _CompileParser(_Parser):
                             self.parent_book, self.parent_locals, *args)
         elif self.token_is_constant():
             # String or number
-            v = self.token()
+            v = unicode(self.token())
             if self.compile_text:
                 tv = v.replace("\\", "\\\\")
                 tv = tv.replace("'", "\\'")
-                self.compile_text += "\targs[%d].append('%s')\n"%(level, tv)
+                self.compile_text += "\targs[%d].append(unicode('%s'))\n"%(level, tv)
             return v
         else:
             self.error(_('expression is not function or constant'))