diff --git a/src/setting/traits.pm b/src/setting/traits.pm
index 45456f6a4b7..496b3b7a472 100644
--- a/src/setting/traits.pm
+++ b/src/setting/traits.pm
@@ -125,7 +125,7 @@ multi trait_mod:<does>(Hash $var, $role) {
     $var does $role;
 }
 
-multi trait_mod:<of>(Code $block is rw, Object $type is rw) {
+multi trait_mod:<of>(Code $block is rw where { .defined }, Object $type is rw) {
     $block does Callable[$type];
 }