Python scipy.optimize.leastsq cov_x is not returned
<pre><code>def letsfit(l,mw,err):
    f=lambda l,p: p[0]*(1+p[1]/l**2+p[2]/l**4)
    pull=lambda p,t,mw,err:(mw-f(l,p))/err
    guess=[1.9,1,1]
    params,cov,info,mess,ierr=opt.leastsq(pull,guess,args=(l,mw,err),full_output=True)
    return params,cov,info,f(l,params),pull(params,l,mw,err)

n=np.array([1.53063882,1.52721648,1.52430138,1.52331225,1.52085916,1.51892116,1.51764307,1.51511098])
en=np.array([0.00016322,0.00016374,0.00016417,0.00016432,0.00016469,0.00016497,0.00016516,0.00012822])
par,co,inf,ff,pp=letsfit(l,nl,enl)
Chiq=(inf['fvec']**2).sum()/(len(l)-3)
</code></pre>

<p>This is my Code. The fit is working fine, but if i try to print 'co' it returns none.</p>