--- src/money.5.c	2015-08-03 04:31:25.000000000 +0900
+++ src/money.6.c	2015-08-03 04:31:25.000000000 +0900
@@ -1,38 +1,45 @@
 #include <stdlib.h>
 #include "money.h"
 
 struct Money
 {
     int amount;
     char *currency;
 };
 
 Money *money_create(int amount, char *currency)
 {
-    Money *m = malloc(sizeof(Money));
+    Money *m;
+
+    if (amount < 0)
+    {
+        return NULL;
+    }
+
+    m = malloc(sizeof(Money));
 
     if (m == NULL)
     {
         return NULL;
     }
 
     m->amount = amount;
     m->currency = currency;
     return m;
 }
 
 int money_amount(Money * m)
 {
     return m->amount;
 }
 
 char *money_currency(Money * m)
 {
     return m->currency;
 }
 
 void money_free(Money * m)
 {
     free(m);
     return;
 }