diff --git a/lib/httpoison/base.ex b/lib/httpoison/base.ex
index eb61119..780d3f1 100644
--- a/lib/httpoison/base.ex
+++ b/lib/httpoison/base.ex
@@ -995,6 +995,7 @@ defmodule HTTPoison.Base do
     {:form,
      Enum.flat_map(body, fn
        {k, [{_k, _v} | _rest] = v} -> flatten_nested_body(v, k)
+       {k, v} when is_map(v) -> flatten_nested_body(v, k)
        {k, v} -> [{k, v}]
      end)}
   end
@@ -1006,6 +1007,9 @@ defmodule HTTPoison.Base do
   defp flatten_nested_body(body, parent_key) do
     flattened_body =
       Enum.reduce(body, [], fn
+        {key, nested_key_values}, acc when is_map(nested_key_values) ->
+          flatten_nested_body(nested_key_values, "#{parent_key}[#{key}]") ++ acc
+
         {key, [{_key, _value} | _rest] = nested_key_values}, acc ->
           flatten_nested_body(nested_key_values, "#{parent_key}[#{key}]") ++ acc
 
diff --git a/test/httpoison_test.exs b/test/httpoison_test.exs
index a898ed4..85e2c68 100644
--- a/test/httpoison_test.exs
+++ b/test/httpoison_test.exs
@@ -82,7 +82,7 @@ defmodule HTTPoisonTest do
     )
   end
 
-  test "post nested form data" do
+  test "post nested form data when given as keyword list" do
     assert_response(
       HTTPoison.post(
         "localhost:4002/post",
@@ -105,6 +105,35 @@ defmodule HTTPoisonTest do
     )
   end
 
+  test "post nested form data when given as map (no order guaranteed)" do
+    assert_response(
+      HTTPoison.post(
+        "localhost:4002/post",
+        {:form, %{not_nested: [1, 2], nested: %{foo: "bar", again: %{hello: "world"}}}},
+        %{"Content-type" => "application/x-www-form-urlencoded"}
+      ),
+      fn %HTTPoison.Response{request: request} = response ->
+        assert {:form, body} = request.body
+
+        assert [{:not_nested, [1, 2]}, {"nested[foo]", "bar"}, {"nested[again][hello]", "world"}]
+               |> MapSet.new()
+               |> MapSet.equal?(MapSet.new(body))
+
+        Regex.match?(~r/"not_nested".*\x01\x02/, response.body)
+        Regex.match?(~r/"nested\[foo\]".*"bar"/, response.body)
+        Regex.match?(~r/"nested\[again\]\[hello\]".*"world"/, response.body)
+
+        assert {:ok,
+                "curl -X POST -H 'Content-type: application/x-www-form-urlencoded'" <> form_params} =
+                 Request.to_curl(response.request)
+
+        assert form_params =~ "-F 'not_nested=\x01\x02'"
+        assert form_params =~ "-F 'nested[foo]=bar'"
+        assert form_params =~ "-F 'nested[again][hello]=world'"
+      end
+    )
+  end
+
   test "put" do
     assert_response(HTTPoison.put("localhost:4002/put", "test"), fn response ->
       assert Request.to_curl(response.request) ==