diff --git a/include/readpassphrase.h b/include/readpassphrase.h
index 031f51796c91..75cb8cb4a1d9 100644
--- a/include/readpassphrase.h
+++ b/include/readpassphrase.h
@@ -39,6 +39,12 @@
 #define RPP_SEVENBIT    0x10		/* Strip the high bit from input. */
 
 #include <sys/cdefs.h>
+#include <sys/_types.h>
+
+#ifndef _SIZE_T_DECLARED
+typedef	__size_t	size_t;
+#define	_SIZE_T_DECLARED
+#endif
 
 __BEGIN_DECLS
 char * readpassphrase(const char *, char *, size_t, int);