diff --git a/sys/kern/subr_prf.c b/sys/kern/subr_prf.c
index dea40344b823..0057d9a7e25e 100644
--- a/sys/kern/subr_prf.c
+++ b/sys/kern/subr_prf.c
@@ -814,7 +814,7 @@ reswitch:	switch (ch = (u_char)*fmt++) {
 			else if (lflag)
 				num = va_arg(ap, long);
 			else if (zflag)
-				num = va_arg(ap, size_t);
+				num = va_arg(ap, ssize_t);
 			else if (hflag)
 				num = (short)va_arg(ap, int);
 			else if (cflag)