diff --git a/poly2tri/common/shapes.h b/poly2tri/common/shapes.h
index d78aa876..1820c02e 100644
--- a/poly2tri/common/shapes.h
+++ b/poly2tri/common/shapes.h
@@ -35,6 +35,7 @@
 
 #include <cmath>
 #include <cstddef>
+#include <iosfwd>
 #include <stdexcept>
 #include <vector>