diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 8e547d2d72c3..dec5818fcfc7 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -615,9 +615,9 @@ expr_list
 
 formals
   : formal ',' formals
-    { $$ = $3; $$->formals.push_back(*$1); }
+    { $$ = $3; $$->formals.emplace_back(*$1); delete $1; }
   | formal
-    { $$ = new ParserFormals; $$->formals.push_back(*$1); $$->ellipsis = false; }
+    { $$ = new ParserFormals; $$->formals.emplace_back(*$1); $$->ellipsis = false; delete $1; }
   |
     { $$ = new ParserFormals; $$->ellipsis = false; }
   | ELLIPSIS