Nested bookparts: fix $papers pop in case of \book block with identifier

diff --git a/lily/lily-parser.cc b/lily/lily-parser.cc
index c46224fbbc596d4de4123b0de2257dadcfa8f93d..afd401baa95fd3b6a2f22b7a528e5f4d8dc789c5 100644 (file)
--- a/lily/lily-parser.cc
+++ b/lily/lily-parser.cc
@@ -257,8 +257,9 @@ push_paper (Lily_parser *parser, Output_def *paper)
 void
 pop_paper (Lily_parser *parser)
 {
-  parser->lexer_->set_identifier (ly_symbol2scm ("$papers"),
-                                  scm_cdr (parser->lexer_->lookup_identifier ("$papers")));
+  if (! scm_is_null (parser->lexer_->lookup_identifier ("$papers")))
+    parser->lexer_->set_identifier (ly_symbol2scm ("$papers"),
+                                    scm_cdr (parser->lexer_->lookup_identifier ("$papers")));
 }
 
 /* Change the paper on top of $papers stack */