diff --git a/src/ast.rs b/src/ast.rs
index 96c1f14..4d91118 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1488,7 +1488,7 @@ where
         clingo_ast_build(
             clingo_ast_type_e_clingo_ast_type_guard as i32,
             &mut ast,
-            operator,
+            operator as isize,
             term.ast,
         )
     } {