diff --git a/sys/amd64/amd64/vm_machdep.c b/sys/amd64/amd64/vm_machdep.c
index 5f4f43c2352..fae37d731b0 100644
--- a/sys/amd64/amd64/vm_machdep.c
+++ b/sys/amd64/amd64/vm_machdep.c
@@ -508,6 +508,9 @@ cpu_set_upcall(struct thread *td, void (*entry)(void *), void *arg,
 		   (((uintptr_t)stack->ss_sp + stack->ss_size - 4) & ~0x0f) - 4;
 		td->td_frame->tf_rip = (uintptr_t)entry;
 
+		/* Return address sentinel value to stop stack unwinding. */
+		suword32((void *)td->td_frame->tf_rsp, 0);
+
 		/* Pass the argument to the entry point. */
 		suword32((void *)(td->td_frame->tf_rsp + sizeof(int32_t)),
 		    (uint32_t)(uintptr_t)arg);
@@ -531,6 +534,9 @@ cpu_set_upcall(struct thread *td, void (*entry)(void *), void *arg,
 	td->td_frame->tf_gs = _ugssel;
 	td->td_frame->tf_flags = TF_HASSEGS;
 
+	/* Return address sentinel value to stop stack unwinding. */
+	suword((void *)td->td_frame->tf_rsp, 0);
+
 	/* Pass the argument to the entry point. */
 	td->td_frame->tf_rdi = (register_t)arg;
 }
diff --git a/sys/i386/i386/vm_machdep.c b/sys/i386/i386/vm_machdep.c
index a0a9473b3bd..562cdfdd756 100644
--- a/sys/i386/i386/vm_machdep.c
+++ b/sys/i386/i386/vm_machdep.c
@@ -524,6 +524,9 @@ cpu_set_upcall(struct thread *td, void (*entry)(void *), void *arg,
 	    (((int)stack->ss_sp + stack->ss_size - 4) & ~0x0f) - 4;
 	td->td_frame->tf_eip = (int)entry;
 
+	/* Return address sentinel value to stop stack unwinding. */
+	suword((void *)td->td_frame->tf_esp, 0);
+
 	/* Pass the argument to the entry point. */
 	suword((void *)(td->td_frame->tf_esp + sizeof(void *)),
 	    (int)arg);