diff --git a/src/library/tactic/tactic_state.h b/src/library/tactic/tactic_state.h
index 1de805d0e2..4fd1fdd480 100644
--- a/src/library/tactic/tactic_state.h
+++ b/src/library/tactic/tactic_state.h
@@ -153,6 +153,7 @@ template<typename T> tactic_state update_option_if_undef(tactic_state const & s,
     return set_options(s, s.get_options().update_if_undef(n, v));
 }
 
+bool is_ts_safe(tactic_state const & s);
 typedef interaction_monad<tactic_state> tactic;
 
 vm_obj to_obj(tactic_state const & s);