diff --git a/library/include/BitArray.h b/library/include/BitArray.h
index dfd187363..5858897a3 100644
--- a/library/include/BitArray.h
+++ b/library/include/BitArray.h
@@ -574,6 +574,8 @@ namespace DFHack
     {
         std::vector<I *> & operator[](O other_id)
         {
+            CHECK_INVALID_ARGUMENT(size_t(other_id) < sizeof(*this) / sizeof(std::vector<I *>));
+
             auto vectors = reinterpret_cast<std::vector<I *> *>(this);
             return vectors[other_id];
         }