diff --git a/poly2tri/sweep/sweep_context.h b/poly2tri/sweep/sweep_context.h
index da9b92a..266408d 100644
--- a/poly2tri/sweep/sweep_context.h
+++ b/poly2tri/sweep/sweep_context.h
@@ -34,6 +34,7 @@
 
 #include <list>
 #include <vector>
+#include <cstddef>
 
 namespace p2t {