Error in `mutate_impl(.data, dots)` when using the combination `mutate_at(m:n, ~ ./...)`

dplyr
tibble

#1
 tibble::tribble(
+      ~Sepal.Length, ~Sepal.Width, ~Petal.Length, ~Petal.Width, ~Species,
+                5.1,          3.5,           1.4,          0.2, "setosa",
+                4.9,            3,           1.4,          0.2, "setosa",
+                4.7,          3.2,           1.3,          0.2, "setosa",
+                4.6,          3.1,           1.5,          0.2, "setosa",
+                  5,          3.6,           1.4,          0.2, "setosa",
+                5.4,          3.9,           1.7,          0.4, "setosa"
+      ) %>% 
+   mutate(row_sum = rowSums(select(., 1:4))) %>% 
+   mutate_at(1:4, ~ ./row_sum)
Error in mutate_impl(.data, dots) : 
  Column `Sepal.Length` must be length 6 (the number of rows) or one, not 150

I just try the mutate_at function on the Stack Overflow example, but I get this weird err info.
Obviously, the rows of th table is 6 instead of 150.

And I try one more example, and get another err info. I feel the combination mutate_at(m:n, ~ ./...) doesn't works always.

> tibble::tribble(
+     ~year, ~month, ~reg_cnt, ~m0_deal, ~m1_deal, ~m2_deal, ~m3_deal, ~m4_deal, ~m5_deal,       ~m0_amount, ~m1_amount, ~m2_amount,       ~m3_amount, ~m4_amount, ~m5_amount,
+     2018L,     1L,      16L,       NA,       NA,       NA,       NA,       NA,       NA,               NA,         NA,         NA,               NA,         NA,         NA,
+     2018L,     2L,   35331L,     441L,       NA,       NA,     151L,       1L,       NA, 1021541.95011338,         NA,         NA, 923841.059602649,      5e+05,         NA,
+     2018L,     3L,    7554L,      77L,       NA,       8L,      21L,       NA,       NA, 993506.493506494,         NA,     937500, 976190.476190476,         NA,         NA,
+     2018L,     4L,    2211L,       NA,      16L,       NA,       NA,       NA,       NA,               NA,     968750,         NA,               NA,         NA,         NA,
+     2018L,     5L,   11070L,    1024L,       1L,       NA,       NA,       NA,       NA,      1025390.625,      5e+05,         NA,               NA,         NA,         NA,
+     2018L,     6L,   11665L,    1107L,       NA,       NA,       NA,       NA,       NA, 1001355.01355014,         NA,         NA,               NA,         NA,         NA
+ ) %>% 
+     
+     mutate_all(~ ifelse(is.na(.),0,.)) %>% 
+     mutate_at(vars(m0_deal:m5_deal), ~ ./reg_cnt)
Error in mutate_impl(.data, dots) : 
  Evaluation error: non-numeric argument to binary operator.

Here is my session info.

> sessionInfo()
R version 3.4.4 (2018-03-15)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

Matrix products: default

locale:
[1] LC_COLLATE=Chinese (Simplified)_People's Republic of China.936  LC_CTYPE=Chinese (Simplified)_People's Republic of China.936   
[3] LC_MONETARY=Chinese (Simplified)_People's Republic of China.936 LC_NUMERIC=C                                                   
[5] LC_TIME=Chinese (Simplified)_People's Republic of China.936    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] bindrcpp_0.2.2      DT_0.4              formattable_0.2.0.1 readxl_1.1.0        pryr_0.1.4          skimr_1.0.2         caret_6.0-79        lattice_0.20-35    
 [9] knitr_1.20          lubridate_1.7.4     forcats_0.3.0       stringr_1.3.0       dplyr_0.7.4         purrr_0.2.4         readr_1.1.1         tidyr_0.8.0        
[17] tibble_1.4.2        ggplot2_2.2.1       tidyverse_1.2.1     RODBC_1.3-15       

loaded via a namespace (and not attached):
 [1] nlme_3.1-137       dimRed_0.1.0       httr_1.3.1         rprojroot_1.3-2    tools_3.4.4        backports_1.1.2    R6_2.2.2           rpart_4.1-13      
 [9] lazyeval_0.2.1     colorspace_1.3-2   nnet_7.3-12        withr_2.1.2        tidyselect_0.2.4   mnormt_1.5-5       compiler_3.4.4     cli_1.0.0         
[17] rvest_0.3.2        xml2_1.2.0         scales_0.5.0       sfsmisc_1.1-2      DEoptimR_1.0-8     psych_1.8.4        robustbase_0.93-0  digest_0.6.15     
[25] foreign_0.8-70     rmarkdown_1.9      pkgconfig_2.0.1    htmltools_0.3.6    htmlwidgets_1.2    rlang_0.2.0        ddalpha_1.3.3      rstudioapi_0.7    
[33] bindr_0.1.1        jsonlite_1.5       ModelMetrics_1.1.0 magrittr_1.5       Matrix_1.2-14      Rcpp_0.12.17       munsell_0.4.3      abind_1.4-5       
[41] stringi_1.1.7      MASS_7.3-50        plyr_1.8.4         recipes_0.1.2      grid_3.4.4         parallel_3.4.4     crayon_1.3.4       haven_1.1.1       
[49] splines_3.4.4      pander_0.6.1       hms_0.4.2          pillar_1.2.2       reshape2_1.4.3     codetools_0.2-15   stats4_3.4.4       CVST_0.2-1        
[57] magic_1.5-8        glue_1.2.0         evaluate_0.10.1    modelr_0.1.2       foreach_1.4.4      cellranger_1.1.0   gtable_0.2.0       kernlab_0.9-26    
[65] assertthat_0.2.0   DRR_0.0.3          gower_0.1.2        prodlim_2018.04.18 broom_0.4.4        class_7.3-14       survival_2.42-3    geometry_0.3-6    
[73] timeDate_3043.102  RcppRoll_0.2.2     iterators_1.0.9    lava_1.6.1         ipred_0.9-6