diff --git a/main.c b/main.c
index d3ce95b..f463792 100644
--- a/main.c
+++ b/main.c
@@ -174,8 +174,11 @@ static void parse_args(int argc, char* const* argv)
         }
 
         default:
-            assert(!"TODO");
+        {
+            printf("Unknown option '%c'!" "\n", opt);
+            exit(1);
             break;
+        }
     }
 
     if (unset_operators)