Error in calculating proportion of reads mapping to mitochondrial transcripts

[allllany]

https://github.com/hbctraining/scRNA-seq/blob/master/lessons/mitoRatio.md
I ran into a problem when I did the last step.
metadata$mtUMI <- Matrix::colSums(counts[which(rownames(counts) %in% mt),], na.rm = T)
error in evaluating the argument 'x' in selecting a method for function 'colSums': object of type 'closure' is not subsettableI am not familiar with the R language, and I have tried many ways to solve it. I am not familiar with the R language, and I have tried many ways to solve it. Hope someone can help me. Thank you.