diff --git a/src/runtime/c/pgf/parser.cxx b/src/runtime/c/pgf/parser.cxx
index 603642add..5b9ba56e3 100644
--- a/src/runtime/c/pgf/parser.cxx
+++ b/src/runtime/c/pgf/parser.cxx
@@ -514,7 +514,7 @@ void PgfAbstractParser::combine(State *state, Item *item, CCat *ccat)
     state->push_item(item);
 }
 
-bool PgfAbstractParser::ItemComparator::operator()(Item *item1, Item *item2)
+bool PgfAbstractParser::ItemComparator::operator()(Item *item1, Item *item2) const
 {
     if (item1->rule.as_object() < item2->rule.as_object())
         return true;
diff --git a/src/runtime/c/pgf/parser.h b/src/runtime/c/pgf/parser.h
index ed7b3baa4..38c5cac10 100644
--- a/src/runtime/c/pgf/parser.h
+++ b/src/runtime/c/pgf/parser.h
@@ -132,7 +132,7 @@ protected:
     } item_prob_comp;
 
     struct ItemComparator : std::less<Item*> {
-        bool operator()(Item *item1, Item *item2);
+        bool operator()(Item *item1, Item *item2) const;
     };
 
     struct Cont {