diff --git a/include/geos/geom/prep/PreparedGeometry.h b/include/geos/geom/prep/PreparedGeometry.h
index 600fca3b0a..6c77513905 100644
--- a/include/geos/geom/prep/PreparedGeometry.h
+++ b/include/geos/geom/prep/PreparedGeometry.h
@@ -20,6 +20,7 @@
 
 #include <vector>
 #include <memory>
+#include <string>
 #include <geos/export.h>
 
 // Forward declarations