diff --git a/lib/mu-msg-part.h b/lib/mu-msg-part.h
index fa2c9d4d..35e5ba80 100644
--- a/lib/mu-msg-part.h
+++ b/lib/mu-msg-part.h
@@ -23,6 +23,7 @@
 #define __MU_MSG_PART_H__
 
 #include <glib.h>
+#include <unistd.h> /* for ssize_t */
 
 G_BEGIN_DECLS