diff --git a/pass/cfg.go b/pass/cfg.go
index 643265c..198208d 100644
--- a/pass/cfg.go
+++ b/pass/cfg.go
@@ -58,7 +58,7 @@ func CFG(fn *ir.Function) error {
 			}
 			target, found := fn.LabelTarget[*lbl]
 			if !found {
-				return errors.New("unknown label")
+				return fmt.Errorf("unknown label %q", *lbl)
 			}
 			cur.Succ = append(cur.Succ, target)
 		}