diff --git a/R/operators.R b/R/operators.R
index 361775f32d3..d65a1087b8a 100644
--- a/R/operators.R
+++ b/R/operators.R
@@ -1161,6 +1161,14 @@ path <- function(...) {
       e2[named]
     )
 
+    # Abort if adding named vertices to a non-empty unnamed graph
+    # Empty graphs have zero vertices
+    if (!is.null(nn) && !is_named(e1) && vcount(e1) > 0) {
+      cli::cli_abort(
+        "Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have {.code NA} names."
+      )
+    }
+
     # When adding vertices via +, all unnamed arguments are interpreted as vertex names of the new vertices.
     res <- add_vertices(e1, nv = vctrs::vec_size_common(!!!e2), attr = e2)
   } else if ("igraph.path" %in% class(e2)) {
@@ -1188,6 +1196,13 @@ path <- function(...) {
     res <- add_vertices(e1, e2)
   } else if (is.character(e2)) {
     ## Adding named vertices
+    # Abort if adding named vertices to a non-empty unnamed graph
+    # Empty graphs have zero vertices
+    if (!is_named(e1) && vcount(e1) > 0) {
+      cli::cli_abort(
+        "Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have {.code NA} names."
+      )
+    }
     res <- add_vertices(e1, length(e2), name = e2)
   } else {
     cli::cli_abort("Cannot add {.obj_type_friendly {type}} to igraph graph.")
diff --git a/tests/testthat/_snaps/operators.md b/tests/testthat/_snaps/operators.md
index c60e2dc767a..a4c0f7e8eb4 100644
--- a/tests/testthat/_snaps/operators.md
+++ b/tests/testthat/_snaps/operators.md
@@ -2,6 +2,10 @@
 
     Can't recycle `name` (size 2) to match `foo` (size 3).
 
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
 # vertices() errors on duplicate attribute names
 
     Duplicate attribute name in `vertices()`: "name".
@@ -18,3 +22,27 @@
 
     Duplicate attribute names in `vertices()`: "foo" and "bar".
 
+# adding named vertices to non-empty unnamed graphs errors
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
+---
+
+    Cannot add named vertices to a non-empty unnamed graph. Existing vertices will have `NA` names.
+
diff --git a/tests/testthat/test-operators.R b/tests/testthat/test-operators.R
index d38e3ae5a1a..c8e48886a0f 100644
--- a/tests/testthat/test-operators.R
+++ b/tests/testthat/test-operators.R
@@ -214,27 +214,28 @@ test_that("t() is aliased to edge reversal for graphs", {
 })
 
 test_that("vertices() works", {
-  g_all_unnamed <- make_empty_graph(1) + vertices("a", "b")
+  # Test adding named vertices to empty graph (0 vertices)
+  g_all_unnamed <- make_empty_graph() + vertices("a", "b")
   expect_s3_class(V(g_all_unnamed), "igraph.vs")
-  expect_identical(V(g_all_unnamed)$name, c(NA, "a", "b"))
+  expect_identical(V(g_all_unnamed)$name, c("a", "b"))
 
-  g_mix_named_unnamed <- make_empty_graph(1) + vertices("a", "b", foo = 5)
+  g_mix_named_unnamed <- make_empty_graph() + vertices("a", "b", foo = 5)
   expect_s3_class(V(g_mix_named_unnamed), "igraph.vs")
-  expect_true(is.na(V(g_mix_named_unnamed)$name[1]))
-  expect_identical(V(g_mix_named_unnamed)$name[-1], c("a", "b"))
-  expect_equal(V(g_mix_named_unnamed)$foo, c(NA, 5, 5))
+  expect_identical(V(g_mix_named_unnamed)$name, c("a", "b"))
+  expect_equal(V(g_mix_named_unnamed)$foo, c(5, 5))
 
-  g_mix_bigger_attribute <- make_empty_graph(1) +
+  g_mix_bigger_attribute <- make_empty_graph() +
     vertices("a", "b", "c", foo = 5:7, bar = 8)
   expect_s3_class(V(g_mix_bigger_attribute), "igraph.vs")
-  expect_identical(V(g_mix_bigger_attribute)$name, c(NA, "a", "b", "c"))
-  expect_equal(V(g_mix_bigger_attribute)$foo, c(NA, 5, 6, 7))
-  expect_equal(V(g_mix_bigger_attribute)$bar, c(NA, 8, 8, 8))
+  expect_identical(V(g_mix_bigger_attribute)$name, c("a", "b", "c"))
+  expect_equal(V(g_mix_bigger_attribute)$foo, c(5, 6, 7))
+  expect_equal(V(g_mix_bigger_attribute)$bar, c(8, 8, 8))
 
-  g_one_unnamed <- make_empty_graph(1) + vertices(letters)
+  g_one_unnamed <- make_empty_graph() + vertices(letters)
   expect_s3_class(V(g_one_unnamed), "igraph.vs")
-  expect_identical(V(g_one_unnamed)$name, c(NA, letters))
+  expect_identical(V(g_one_unnamed)$name, letters)
 
+  # Test adding unnamed vertices (attributes only) - should work with any graph
   g_all_named <- make_empty_graph(1) + vertices(foo = 5:7)
   expect_s3_class(V(g_all_named), "igraph.vs")
   expect_null(V(g_all_named)$name)
@@ -249,7 +250,15 @@ test_that("vertices() works", {
   expect_s3_class(V(g_none), "igraph.vs")
   expect_null(V(g_none)$name)
 
-  expect_snapshot_error(make_empty_graph(1) + vertices("a", "b", foo = 5:7))
+  # Test that adding named vertices to non-empty graph with mismatched attributes errors
+  expect_snapshot_error(
+    make_empty_graph() + vertices("a", "b", foo = 5:7)
+  )
+
+  # Test that adding named vertices to non-empty unnamed graph errors
+  expect_snapshot_error(
+    make_empty_graph(1) + vertices("a", "b")
+  )
 })
 
 test_that("vertices() errors on duplicate attribute names", {
@@ -274,6 +283,47 @@ test_that("vertices() errors on duplicate attribute names", {
   )
 })
 
+test_that("adding named vertices to non-empty unnamed graphs errors", {
+  # Test with vertex() function
+  expect_snapshot_error(
+    make_ring(10) + vertex(1)
+  )
+
+  expect_snapshot_error(
+    make_ring(10) + vertex("a")
+  )
+
+  expect_snapshot_error(
+    make_ring(10) + vertices("a", "b")
+  )
+
+  # Test with character vector
+  expect_snapshot_error(
+    make_ring(10) + c("a", "b")
+  )
+
+  # No error when adding to named graph
+  g <- make_ring(10)
+  V(g)$name <- letters[1:10]
+  expect_no_error(g + vertex("k"))
+  expect_no_error(g + c("x", "y"))
+
+  # No error when adding unnamed vertices
+  expect_no_error(make_ring(10) + vertex(foo = 5))
+  expect_no_error(make_ring(10) + vertices(foo = 1:3))
+  expect_no_error(make_ring(10) + 5)
+
+  # No error when adding to empty graph (0 vertices)
+  expect_no_error(make_empty_graph() + vertex("a"))
+  expect_no_error(make_empty_graph() + c("a", "b"))
+  expect_no_error(make_empty_graph() + vertices("a", "b"))
+
+  # Error when adding to non-empty unnamed graph (even with 1 vertex)
+  expect_snapshot_error(make_empty_graph(1) + vertex("a"))
+  expect_snapshot_error(make_empty_graph(1) + c("a", "b"))
+})
+
+
 test_that("infix operators work", {
   g <- make_ring(10)
   V(g)$name <- letters[1:10]