redis hmset and set is not equviant

e.g.
a=dict(a=1,b=2,c=3)
r=redis.Redis()
r.set("haha",a)
r.hget("haha","b")#this will raise a exception

the right way is as follows:
r.hmset("haha",a)#attenation:when you use this approach,if you use r.set("haha",a) first,this also will occur a exception
r.hget("haha","b")