diff --git a/include/minix/portio.h b/include/minix/portio.h
index efb8c75e4..4682a7b51 100644
--- a/include/minix/portio.h
+++ b/include/minix/portio.h
@@ -13,7 +13,7 @@ Created:	Jan 15, 1992 by Philip Homburg
 
 unsigned inb(u16_t _port);
 unsigned inw(u16_t _port);
-unsigned inl(u32_t _port);
+unsigned inl(u16_t _port);
 void outb(u16_t _port, u8_t _value);
 void outw(u16_t _port, u16_t _value);
 void outl(u16_t _port, u32_t _value);