diff --git a/eval.go b/eval.go
index a77fff4..f13c90f 100644
--- a/eval.go
+++ b/eval.go
@@ -87,6 +87,8 @@ func (c *Context) Exec(w Word) error {
 		if err != nil {
 			return err
 		}
+	} else {
+		return fmt.Errorf("unable to parse word %s", w.Name)
 	}
 	return nil
 }