diff --git a/arch/arm/core/cortex_m/thread.c b/arch/arm/core/cortex_m/thread.c
index 4cadc4e72bf..fa500032d3c 100644
--- a/arch/arm/core/cortex_m/thread.c
+++ b/arch/arm/core/cortex_m/thread.c
@@ -586,8 +586,8 @@ void arch_switch_to_main_thread(struct k_thread *main_thread, char *stack_ptr,
 	"mov   r3, #0\n"
 	"ldr   r4, =z_thread_entry\n"
 	"bx    r4\n"		/* We don’t intend to return, so there is no need to link. */
-	:
-	: "r" (_main), "r" (stack_ptr)
+	: "+r" (_main)
+	: "r" (stack_ptr)
 	: "r0", "r1", "r2", "r3", "r4", "ip", "lr");
 
 	CODE_UNREACHABLE;