diff --git a/src/runtime/internal/transitions.ts b/src/runtime/internal/transitions.ts
index 21d67a12ba..2fc81cc21f 100644
--- a/src/runtime/internal/transitions.ts
+++ b/src/runtime/internal/transitions.ts
@@ -48,7 +48,7 @@ export function transition_in(block, local?: 0 | 1) {
 	}
 }
 
-export function transition_out(block, local: 0 | 1, detach: 0 | 1, callback) {
+export function transition_out(block, local: 0 | 1, detach?: 0 | 1, callback?) {
 	if (block && block.o) {
 		if (outroing.has(block)) return;
 		outroing.add(block);