diff --git a/thread_pthread.c b/thread_pthread.c
index 451f47e07f..5c181556b9 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -1052,7 +1052,7 @@ static void *
 thread_start_func_1(void *th_ptr)
 {
     rb_thread_t *th = th_ptr;
-    RB_ALTSTACK_INIT(void *altstack, th->altstack);
+    RB_ALTSTACK_INIT(void *altstack, th->nt->altstack);
 #if USE_THREAD_CACHE
   thread_start:
 #endif
@@ -1208,7 +1208,7 @@ native_thread_create(rb_thread_t *th)
 	const size_t space = space_size(stack_size);
 
 #ifdef USE_SIGALTSTACK
-        th->altstack = rb_allocate_sigaltstack();
+        th->nt->altstack = rb_allocate_sigaltstack();
 #endif
         th->ec->machine.stack_maxsize = stack_size - space;
 
diff --git a/thread_pthread.h b/thread_pthread.h
index 8e8c1eb43d..ebca71b5cd 100644
--- a/thread_pthread.h
+++ b/thread_pthread.h
@@ -50,6 +50,10 @@ struct rb_native_thread {
         rb_nativethread_cond_t intr; /* th->interrupt_lock */
         rb_nativethread_cond_t readyq; /* use sched->lock */
     } cond;
+
+#ifdef USE_SIGALTSTACK
+    void *altstack;
+#endif
 };
 
 #undef except
diff --git a/vm_core.h b/vm_core.h
index 58fb1e7e3b..2fca0aecd2 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -101,9 +101,6 @@ extern int ruby_assert_critical_section_entered;
 #include "vm_opts.h"
 
 #include "ruby/thread_native.h"
-#include THREAD_IMPL_H
-
-#define RUBY_VM_THREAD_MODEL 2
 
 /*
  * implementation selector of get_insn_info algorithm
@@ -162,6 +159,9 @@ void *rb_register_sigaltstack(void *);
 #  define RB_ALTSTACK(var) (0)
 #endif
 
+#include THREAD_IMPL_H
+#define RUBY_VM_THREAD_MODEL 2
+
 /*****************/
 /* configuration */
 /*****************/
@@ -1072,10 +1072,6 @@ typedef struct rb_thread_struct {
     VALUE name;
 
     struct rb_ext_config ext_config;
-
-#ifdef USE_SIGALTSTACK
-    void *altstack;
-#endif
 } rb_thread_t;
 
 static inline unsigned int