diff --git a/Chapter4/Exercise 4-04/stack.c b/Chapter4/Exercise 4-04/stack.c
index 6ea7ad3..6c03606 100644
--- a/Chapter4/Exercise 4-04/stack.c	
+++ b/Chapter4/Exercise 4-04/stack.c	
@@ -76,15 +76,21 @@ void duplicate(void)
 
 /* swap:  swap the top two elements of the stack */
 void swap(void)
-{
-    double top1 = pop();
-    double top2 = pop();
-    push(top1);
-    push(top2);
+ {
+    double top1;
+    double top2;
+    if (top > 1) {
+        top1 = pop();
+        top2 = pop();
+        push(top1);
+        push(top2);
+    }
+    else
+        printf("erorr: there is only one or no element in stack\n");
 }
 
 /* clear: clear stack */
 void clear(void)
 {
     sp = 0;
-}
\ No newline at end of file
+}