1 files changed, 3 insertions, 1 deletions
diff --git a/033length.cc b/033length.cc
index 2268611a..8f18180e 100644
--- a/033length.cc
+++ b/033length.cc
@@ -23,6 +23,8 @@ case LENGTH: {
     break;
   }
   products.resize(1);
-  products.at(0).push_back(Memory[x.value]);
+  assert(is_integer(Memory[x.value]));
+  assert(!is_negative(Memory[x.value]));
+  products.at(0).push_back(Memory[x.value]);  // length must be a positive integer
   break;
 }