diff --git a/lib/src/metta/runner/stdlib/debug.rs b/lib/src/metta/runner/stdlib/debug.rs
index 326b04ab..64af9264 100644
--- a/lib/src/metta/runner/stdlib/debug.rs
+++ b/lib/src/metta/runner/stdlib/debug.rs
@@ -58,7 +58,7 @@ grounded_op!(TraceOp, "trace!");
 
 impl Grounded for TraceOp {
     fn type_(&self) -> Atom {
-        Atom::expr([ARROW_SYMBOL, ATOM_TYPE_UNDEFINED, Atom::var("a"), Atom::var("a")])
+        Atom::expr([ARROW_SYMBOL, ATOM_TYPE_UNDEFINED, ATOM_TYPE_ATOM, ATOM_TYPE_ATOM])
     }
 
     fn as_execute(&self) -> Option<&dyn CustomExecute> {