diff --git a/src/nvim/extmark.c b/src/nvim/extmark.c
index 60b7b024f1..4b2dccd8a4 100644
--- a/src/nvim/extmark.c
+++ b/src/nvim/extmark.c
@@ -268,7 +268,7 @@ bool extmark_clear(buf_T *buf, uint64_t ns_id,
       }
       ExtmarkNs *my_ns = all_ns ? buf_ns_ref(buf, item.ns_id, false) : ns;
       map_del(uint64_t, uint64_t)(my_ns->map, item.mark_id);
-      map_del(uint64_t, ExtmarkItem)(buf->b_extmark_index, mark.id);
+      map_del(uint64_t, ExtmarkItem)(buf->b_extmark_index, start_id);
       marktree_del_itr(buf->b_marktree, itr, false);
     } else {
       marktree_itr_next(buf->b_marktree, itr);
diff --git a/test/functional/ui/decorations_spec.lua b/test/functional/ui/decorations_spec.lua
index 98aafd8757..0e4b3574e9 100644
--- a/test/functional/ui/decorations_spec.lua
+++ b/test/functional/ui/decorations_spec.lua
@@ -697,4 +697,50 @@ end]]
                                                         |
     ]]}
   end)
+  it('does not crash when deleting a cleared buffer #15212', function()
+    exec_lua [[
+      ns = vim.api.nvim_create_namespace("myplugin")
+      vim.api.nvim_buf_set_extmark(0, ns, 0, 0, {virt_text = {{"a"}}, end_col = 0})
+    ]]
+    screen:expect{grid=[[
+      ^ a                                                |
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+                                                        |
+    ]]}
+
+    exec_lua [[
+      vim.api.nvim_buf_clear_namespace(0, ns, 0, -1)
+      vim.cmd("bdelete")
+    ]]
+    screen:expect{grid=[[
+      ^                                                  |
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+      {1:~                                                 }|
+                                                        |
+    ]]}
+    helpers.assert_alive()
+  end)
 end)