Yum update after f17 -> f18 fedup via iso

[JOYCE POLZIN]

sudo yum update 
Loaded plugins: langpacks, presto, refresh-packagekit 
Error: Package tuple ('libpng-compat', 'x86_64', '2', '1.5.10', '1.fc17') could not be found in rpmdb 


What's up with this?? 
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