diff --git a/chopstx-cortex-m.c b/chopstx-cortex-m.c
index 89315a3..4a31af2 100644
--- a/chopstx-cortex-m.c
+++ b/chopstx-cortex-m.c
@@ -575,6 +575,7 @@ preempt (struct chx_thread * tp_next)
 	"ldr	r1, [r0], #4\n\t"
 	"msr	PSP, r1\n\t"
 #endif
+	"mov	r0, #0\n\t"
 	/* Unmask interrupts.  */
 #if defined(__ARM_ARCH_6M__)
 	"cpsie	i\n\t"