diff --git a/runtime/lua/vim/treesitter/_range.lua b/runtime/lua/vim/treesitter/_range.lua
index f4db5016ac..35081c6400 100644
--- a/runtime/lua/vim/treesitter/_range.lua
+++ b/runtime/lua/vim/treesitter/_range.lua
@@ -143,6 +143,29 @@ function M.contains(r1, r2)
   return true
 end
 
+--- @param source integer|string
+--- @param index integer
+--- @return integer
+local function get_offset(source, index)
+  if index == 0 then
+    return 0
+  end
+
+  if type(source) == 'number' then
+    return api.nvim_buf_get_offset(source, index)
+  end
+
+  local byte = 0
+  local next_offset = source:gmatch('()\n')
+  local line = 1
+  while line <= index do
+    byte = next_offset() --[[@as integer]]
+    line = line + 1
+  end
+
+  return byte
+end
+
 ---@private
 ---@param source integer|string
 ---@param range Range
@@ -152,19 +175,10 @@ function M.add_bytes(source, range)
     return range --[[@as Range6]]
   end
 
-  local start_row, start_col, end_row, end_col = range[1], range[2], range[3], range[4]
-  local start_byte = 0
-  local end_byte = 0
+  local start_row, start_col, end_row, end_col = M.unpack4(range)
   -- TODO(vigoux): proper byte computation here, and account for EOL ?
-  if type(source) == 'number' then
-    -- Easy case, this is a buffer parser
-    start_byte = api.nvim_buf_get_offset(source, start_row) + start_col
-    end_byte = api.nvim_buf_get_offset(source, end_row) + end_col
-  elseif type(source) == 'string' then
-    -- string parser, single `\n` delimited string
-    start_byte = vim.fn.byteidx(source, start_col)
-    end_byte = vim.fn.byteidx(source, end_col)
-  end
+  local start_byte = get_offset(source, start_row) + start_col
+  local end_byte = get_offset(source, end_row) + end_col
 
   return { start_row, start_col, start_byte, end_row, end_col, end_byte }
 end
diff --git a/test/functional/treesitter/parser_spec.lua b/test/functional/treesitter/parser_spec.lua
index 60042ab2fd..14de07639b 100644
--- a/test/functional/treesitter/parser_spec.lua
+++ b/test/functional/treesitter/parser_spec.lua
@@ -486,7 +486,6 @@ end]]
     eq({ 'any-of?', 'contains?', 'eq?', 'has-ancestor?', 'has-parent?', 'is-main?', 'lua-match?', 'match?', 'vim-match?' }, res_list)
   end)
 
-
   it('allows to set simple ranges', function()
     insert(test_text)
 
@@ -528,6 +527,7 @@ end]]
 
     eq(range_tbl, { { { 0, 0, 0, 17, 1, 508 } } })
   end)
+
   it("allows to set complex ranges", function()
     insert(test_text)
 
@@ -992,4 +992,58 @@ int x = INT_MAX;
     }, run_query())
 
   end)
+
+  it('handles ranges when source is a multiline string (#20419)', function()
+    local source = [==[
+      vim.cmd[[
+        set number
+        set cmdheight=2
+        set lastsatus=2
+      ]]
+
+      set query = [[;; query
+        ((function_call
+          name: [
+            (identifier) @_cdef_identifier
+            (_ _ (identifier) @_cdef_identifier)
+          ]
+          arguments: (arguments (string content: _ @injection.content)))
+          (#set! injection.language "c")
+          (#eq? @_cdef_identifier "cdef"))
+      ]]
+    ]==]
+
+    local r = exec_lua([[
+      local parser = vim.treesitter.get_string_parser(..., 'lua')
+      parser:parse()
+      local ranges = {}
+      parser:for_each_tree(function(tstree, tree)
+        ranges[tree:lang()] = { tstree:root():range(true) }
+      end)
+      return ranges
+    ]], source)
+
+    eq({
+      lua = { 0, 6, 6, 16, 4, 438 },
+      query = { 6, 20, 113, 15, 6, 431 },
+      vim = { 1, 0, 16, 4, 6, 89 }
+    }, r)
+
+    -- The above ranges are provided directly from treesitter, however query directives may mutate
+    -- the ranges but only provide a Range4. Strip the byte entries from the ranges and make sure
+    -- add_bytes() produces the same result.
+
+    local rb = exec_lua([[
+      local r, source = ...
+      local add_bytes = require('vim.treesitter._range').add_bytes
+      for lang, range in pairs(r) do
+        r[lang] = {range[1], range[2], range[4], range[5]}
+        r[lang] = add_bytes(source, r[lang])
+      end
+      return r
+    ]], r, source)
+
+    eq(rb, r)
+
+  end)
 end)