diff --git a/src/core/corelib.c b/src/core/corelib.c
index a702e48e..07b7f40d 100644
--- a/src/core/corelib.c
+++ b/src/core/corelib.c
@@ -715,6 +715,7 @@ JANET_CORE_FN(janet_core_sandbox,
             }
             opt++;
         }
+        if (opt->name == NULL) janet_panicf("unknown capability %v", argv[i]);
     }
     janet_sandbox(flags);
     return janet_wrap_nil();