diff --git a/sys/alpha/alpha/machdep.c b/sys/alpha/alpha/machdep.c
index 668042597ae..d43b841f5e5 100644
--- a/sys/alpha/alpha/machdep.c
+++ b/sys/alpha/alpha/machdep.c
@@ -1329,6 +1329,7 @@ sendsig(sig_t catcher, int sig, sigset_t *mask, u_long code)
 	    frame->tf_regs[FRAME_TRAPARG_A1];
 	sf.sf_uc.uc_mcontext.mc_regs[R_TRAPARG_A2] =
 	    frame->tf_regs[FRAME_TRAPARG_A2];
+	sf.sf_uc.uc_mcontext.mc_format = __UC_REV0_SIGFRAME;
 
 	/*
 	 * Allocate and validate space for the signal handler
diff --git a/sys/alpha/include/ucontext.h b/sys/alpha/include/ucontext.h
index 50b1418c391..3e888814385 100644
--- a/sys/alpha/include/ucontext.h
+++ b/sys/alpha/include/ucontext.h
@@ -44,7 +44,10 @@ typedef struct __mcontext {
 	unsigned long mc_fpcr;
 	unsigned long mc_fp_control;
 	long	mc_ownedfp;
-	long	__spare__[7];
+#define	__UC_REV0_SIGFRAME	1	/* context is a signal frame */
+#define	__UC_REV0_TRAPFRAME	2	/* context is a trap frame */
+	long	mc_format;
+	long	__spare__[6];
 } mcontext_t;
 
 #endif /* !_MACHINE_UCONTEXT_H_ */