diff --git a/R/flow.R b/R/flow.R
index 8a852f0f58..46ba499364 100644
--- a/R/flow.R
+++ b/R/flow.R
@@ -738,10 +738,12 @@ st_min_cuts <- all_st_mincuts_impl
 dominator_tree <- function(graph, root, mode = c("out", "in", "all", "total")) {
   # Argument checks
   ensure_igraph(graph)
-  root <- as_igraph_vs(graph, root)
-  if (length(root) == 0) {
+
+  if (missing(root) || is.null(root)) {
     cli::cli_abort("{.arg root} must be specified.")
   }
+  root <- as_igraph_vs(graph, root)
+
   mode <- switch(igraph.match.arg(mode),
     "out" = 1,
     "in" = 2,
diff --git a/tests/testthat/_snaps/flow.md b/tests/testthat/_snaps/flow.md
index bf24e513c6..01e1e52f55 100644
--- a/tests/testthat/_snaps/flow.md
+++ b/tests/testthat/_snaps/flow.md
@@ -86,7 +86,7 @@
       dominator_tree(g_tree)
     Condition
       Error in `dominator_tree()`:
-      ! argument "root" is missing, with no default
+      ! `root` must be specified.
 
 ---