diff --git a/components/finsh/shell.c b/components/finsh/shell.c
index 462d4643e5c5565f3ee7e636b5b021959805dd9a..8e1f814560c5dee6485cf465db8b82026de75721 100644
--- a/components/finsh/shell.c
+++ b/components/finsh/shell.c
@@ -84,13 +84,12 @@ const char *finsh_get_prompt()
 
 static char finsh_getchar(void)
 {
-    RT_ASSERT(shell != RT_NULL);
-
 #ifdef RT_USING_POSIX_STDIN
     return getchar();
 #else
     char ch;
 
+    RT_ASSERT(shell != RT_NULL);
     while (rt_device_read(shell->device, -1, &ch, 1) != 1)
         rt_sem_take(&shell->rx_sem, RT_WAITING_FOREVER);