check response format error

lib/open_graph.ex

@@ -46,8 +46,14 @@ defmodule OpenGraph do
     |> handle_response()
   end
 
-  defp handle_response({:ok, %Req.Response{status: status} = response}) when status in 200..299 do
-    {:ok, parse(response.body)}
+  defp handle_response({:ok, %Req.Response{status: status, body: body}})
+       when status in 200..299 and is_binary(body) do
+    {:ok, parse(body)}
+  end
+
+  defp handle_response({:ok, %Req.Response{status: status, body: body}})
+       when status in 200..299 do
+    {:error, %OpenGraph.Error{reason: {:unexpected_format, body}}}
   end
 
   defp handle_response({:ok, %Req.Response{status: status}}) when status in 300..399 do

lib/open_graph/error.ex

@@ -23,6 +23,10 @@ defmodule OpenGraph.Error do
     "unexpected response is received. HTTP status code: #{status_code}"
   end
 
+  defp format_reason({:unexpected_format, body}) do
+    "unexpected response format is received. body: #{inspect(body)}"
+  end
+
   defp format_reason({:request_error, reason}) do
     "request error. reason: #{reason}"
   end

test/open_graph_test.exs

@@ -97,6 +97,17 @@ defmodule OpenGraphTest do
     end
   end
 
+  test "fetch!/1 raises exception for unexpected response format" do
+    data = %{"title" => "The Rock"}
+    Req.Test.stub(MyStub, &Req.Test.json(&1, data))
+
+    assert_raise Error,
+                 "unexpected response format is received. body: #{inspect(data)}",
+                 fn ->
+                   fetch!("https://example.com/")
+                 end
+  end
+
   test "fetch!/1 raises exception for server downtime" do
     Req.Test.stub(MyStub, &Req.Test.transport_error(&1, :econnrefused))