I have a image from contourf that looks like this:

It is made from this code snippet:
fig = plt.figure()
Z = uavg_lon[:,1,:].transpose()
plt.title('ERA-interim 100hpa U average in [-20,20]',fontsize=16)
c0 = plt.contourf( lon , range(4*12) , Z , cmap = cm.bwr , levels =
range(-25,30,5))
#plt.clabel(c0, fmt='%2d', inline=True)
plt.xlim([-180,180])
plt.xlabel('longitude',fontsize=14)
plt.xticks(np.arange(-180,180+30,30),fontsize=14)
plt.yticks(monthlabellocs,monthlabels,fontsize=14)
plt.ylim([0,48])
plt.xlim([-180,180])
fig.set_size_inches(8,10)
fn = 'hovmol_100u.pdf'
fig.savefig(fn , format='pdf' , bbox_inches='tight')
When I try to add contour labels with the clabel (by uncommenting the commented line), I get this:

I cannot find any examples online that describe this problem. What is happening, and how can I fix this?