diff --git a/thread.c b/thread.c
index 27a6ffb955..9314cd5499 100644
--- a/thread.c
+++ b/thread.c
@@ -639,8 +639,6 @@ void rb_ec_clear_current_thread_trace_func(const rb_execution_context_t *ec);
 static int
 thread_start_func_2(rb_thread_t *th, VALUE *stack_start)
 {
-    STACK_GROW_DIR_DETECTION;
-
     RUBY_DEBUG_LOG("th:%u", rb_th_serial(th));
     VM_ASSERT(th != th->vm->ractor.main_thread);
 
diff --git a/thread_pthread.c b/thread_pthread.c
index 1b51872199..332e50f6e1 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -1989,7 +1989,7 @@ native_thread_init_main_thread_stack(void *addr)
 #else
     if (!native_main_thread.stack_start ||
         STACK_UPPER((VALUE *)(void *)&addr,
-                    native_main_thread.stack_start > addr,
+                    native_main_thread.stack_start > (VALUE *)addr,
                     native_main_thread.stack_start < (VALUE *)addr)) {
         native_main_thread.stack_start = (VALUE *)addr;
     }