diff --git a/sys/amd64/include/in_cksum.h b/sys/amd64/include/in_cksum.h
--- a/sys/amd64/include/in_cksum.h
+++ b/sys/amd64/include/in_cksum.h
@@ -37,10 +37,6 @@
 #ifndef _MACHINE_IN_CKSUM_H_
 #define	_MACHINE_IN_CKSUM_H_	1
 
-#ifndef _SYS_CDEFS_H_
-#error this file needs sys/cdefs.h as a prerequisite
-#endif
-
 #include <sys/cdefs.h>
 
 #define in_cksum(m, len)	in_cksum_skip(m, len, 0)
diff --git a/sys/i386/include/in_cksum.h b/sys/i386/include/in_cksum.h
--- a/sys/i386/include/in_cksum.h
+++ b/sys/i386/include/in_cksum.h
@@ -37,10 +37,6 @@
 #ifndef _MACHINE_IN_CKSUM_H_
 #define	_MACHINE_IN_CKSUM_H_	1
 
-#ifndef _SYS_CDEFS_H_
-#error this file needs sys/cdefs.h as a prerequisite
-#endif
-
 #include <sys/cdefs.h>
 
 #define in_cksum(m, len)	in_cksum_skip(m, len, 0)