diff --git a/types/output/output.c b/types/output/output.c
index a352a5e59..5a4c3b666 100644
--- a/types/output/output.c
+++ b/types/output/output.c
@@ -670,6 +670,7 @@ static bool output_basic_test(struct wlr_output *output,
 		}
 
 		for (size_t i = 0; i < state->layers_len; i++) {
+			assert(state->layers[i].layer != NULL);
 			state->layers[i].accepted = false;
 		}
 	}