diff --git a/libraries/rtos/rtx/TARGET_ARM7/cmsis_os.h b/libraries/rtos/rtx/TARGET_ARM7/cmsis_os.h
index ae92bfd548..314a5788c1 100644
--- a/libraries/rtos/rtx/TARGET_ARM7/cmsis_os.h
+++ b/libraries/rtos/rtx/TARGET_ARM7/cmsis_os.h
@@ -243,7 +243,7 @@ typedef struct os_thread_def  {
   os_pthread               pthread;      ///< start address of thread function
   osPriority             tpriority;      ///< initial thread priority
   uint32_t               stacksize;      ///< stack size requirements in bytes
-  unsigned char         *stack_pointer;  ///< pointer to the stack memory block
+  uint32_t              *stack_pointer;  ///< pointer to the stack memory block
   struct OS_TCB          tcb;
 } osThreadDef_t;
 
@@ -337,7 +337,7 @@ int32_t osKernelRunning(void);
 extern osThreadDef_t os_thread_def_##name
 #else                            // define the object
 #define osThreadDef(name, priority, stacksz)  \
-unsigned char os_thread_def_stack_##name [stacksz] __attribute__((aligned (4))); \
+uint32_t os_thread_def_stack_##name [stacksz / sizeof(uint32_t)]; \
 osThreadDef_t os_thread_def_##name = \
 { (name), (priority), (stacksz), (os_thread_def_stack_##name)}
 #endif