'Apply' giving me errors

So i have a simple function:

bmass=function(y){
weight=y$WT*y$MSTR
return(bio)
}

And want to apply to a whole bunch of rows in my data.frame:

final1=apply(final,1,yldbu)


BUT...recieve the following error:
"Error in y$WT : $ operator is invalid for atomic vectors"


However when i try:

final[1,]$WT*final[1,]$MSTR

[1] 156.3


It gives me the correct answer....what is apply not liking in my code?

Thanks



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