diff --git a/lib/types.hpp b/lib/types.hpp
index 7871280..0667b06 100644
--- a/lib/types.hpp
+++ b/lib/types.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <stddef.h> // for size_t
 #include <vector>
 
 // We use size_ts for fast indexing. Note that there is little type safety here