R - can't convert from

Question

I have a vector list of data frames I generated from a normal data frame as follows. Gene expression contains columns identifying each observation's cell line, gene, chromosomal region, and expression level -- multiple genes may belong to a specific chromosomal region and I want to compare expression in one chromosomal region against all others. It looks something like this:

gene    region    cell_line    expression
A       X         Joe          1
B       X         Joe          2 
C       Y         Joe          2
D       Z         Joe          3
E       Z         Joe          0
A       X         Claire       2
B       X         Claire       1
C       Y         Claire       3
D       Z         Claire       3
E       Z         Claire       1

I split it based on cell line.

gene_expression_groups <- gene_expression %>% 
  group_by(cell_line) %>% 
  group_split()

Next, I try to get the mean expression of all chromosomal regions other than that in the one in question for all regions via

for (i in seq_along(gene_expression_groups)){
  gene_expression_groups[[i]] <- as_tibble(gene_expression_groups[[i]]) %>% 
    group_by(chromosomal_region) %>%
    group_modify(~anti_join(gene_expression_groups[[i]], .) %>% 
                   summarize(muG = mean(expression),
                             sigma2G = var(expression),
                             NG = n()))
}

Ideally the end product would be a new vector list, where each split group now looks like so.

region    cell_line     mean_other    standard_deviation_other   
X         Joe           1.67          some number
Y         Joe           1.5           some number
Z         Joe           1.67          some number

Unfortunately, this code only results in "Error: can't convert from <grouped_df< to <tbl_df< due to loss of precision" and I'm lost as to what to do. Any help would be appreciated!