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Bug in Sum causes index variable to remain set?

Hello, I have found what appears to be a bug in Sum in Mathematica 4.0.  
I stumbled across is trying to sum an expression with subscripts.  

In[1]:= n = 1;

In[2]:= Sum[1, {f[x, 1], 1, 3}]
Out[2]= 3
In[3]:= ?f
In[4]:= Sum[1, {f[x, n], 1, 3}]
Out[4]= 3
In[5]:= ?f

In[6]:= Clear[f];
In[7]:= Sum[1, {f[x, 1*1], 1, 3}]
Out[7]= 3
In[8]:= ?f

The way that f gets set by the Sum to the final value is a bug is it 
not?  I would guess that Sum unsets the literal expression f[x, n] or 
f[x, 1*1], rather than exactly what it set in the first place.  I can 
get around this by f[x,n]=. whenever I use it... but I would rather 
not have a line with a comment saying "Mathematica bug work-around".  
Integrate works fine under similar circumstances.


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