diff --git a/R-package/R/utils.R b/R-package/R/utils.R
index 3f67ff23c..d08a411e2 100644
--- a/R-package/R/utils.R
+++ b/R-package/R/utils.R
@@ -104,7 +104,7 @@ check.booster.params <- function(params, ...) {
 
   # for multiclass, expect num_class to be set
   if (typeof(params[['objective']]) == "character" &&
-      substr(NVL(params[['objective']], 'x'), 1, 6) == 'multi:' &&
+      startsWith(NVL(params[['objective']], 'x'), 'multi:') &&
       as.numeric(NVL(params[['num_class']], 0)) < 2) {
         stop("'num_class' > 1 parameter must be set for multiclass classification")
   }
diff --git a/R-package/R/xgb.model.dt.tree.R b/R-package/R/xgb.model.dt.tree.R
index 73cdecc5c..eb64b1eab 100644
--- a/R-package/R/xgb.model.dt.tree.R
+++ b/R-package/R/xgb.model.dt.tree.R
@@ -109,7 +109,7 @@ xgb.model.dt.tree <- function(model = NULL, text = NULL,
   } else {
     trees <- trees[trees >= 0 & trees <= max(td$Tree)]
   }
-  td <- td[Tree %in% trees & !grepl('^booster', t)]
+  td <- td[Tree %in% trees & !is.na(t) & !startsWith(t, 'booster')]
 
   td[, Node := as.integer(sub("^([0-9]+):.*", "\\1", t))]
   if (!use_int_id) td[, ID := add.tree.id(Node, Tree)]
diff --git a/tests/ci_build/lint_r.R b/tests/ci_build/lint_r.R
index 9b55ebfce..3d059262e 100644
--- a/tests/ci_build/lint_r.R
+++ b/tests/ci_build/lint_r.R
@@ -43,6 +43,7 @@ my_linters <- list(
   spaces_inside_linter = lintr::spaces_inside_linter(),
   spaces_left_parentheses_linter = lintr::spaces_left_parentheses_linter(),
   sprintf = lintr::sprintf_linter(),
+  string_boundary = lintr::string_boundary_linter(),
   trailing_blank_lines_linter = lintr::trailing_blank_lines_linter(),
   trailing_whitespace_linter = lintr::trailing_whitespace_linter(),
   true_false = lintr::T_and_F_symbol_linter(),