diff --git a/include/libdivecomputer/iostream.h b/include/libdivecomputer/iostream.h
index e2cb439..d7d2621 100644
--- a/include/libdivecomputer/iostream.h
+++ b/include/libdivecomputer/iostream.h
@@ -22,6 +22,7 @@
 #ifndef DC_IOSTREAM_H
 #define DC_IOSTREAM_H
 
+#include <stddef.h>
 #include <libdivecomputer/common.h>
 #include <libdivecomputer/context.h>