diff --git a/lib/tesla/adapter/hackney.ex b/lib/tesla/adapter/hackney.ex
index e5965857..3d080787 100644
--- a/lib/tesla/adapter/hackney.ex
+++ b/lib/tesla/adapter/hackney.ex
@@ -72,7 +72,7 @@ if Code.ensure_loaded?(:hackney) do
     end
 
     defp request(method, url, headers, body, opts) do
-      handle(:hackney.request(method, url, headers, body || '', opts), opts)
+      handle(:hackney.request(method, url, headers, body || ~c"", opts), opts)
     end
 
     defp request_stream(method, url, headers, body, opts) do
diff --git a/lib/tesla/adapter/httpc.ex b/lib/tesla/adapter/httpc.ex
index 5d79a6e1..ebaff440 100644
--- a/lib/tesla/adapter/httpc.ex
+++ b/lib/tesla/adapter/httpc.ex
@@ -80,8 +80,8 @@ defmodule Tesla.Adapter.Httpc do
     headers = for {key, value} <- headers, do: {to_charlist(key), to_charlist(value)}
 
     {content_type, headers} =
-      case List.keytake(headers, 'content-type', 0) do
-        nil -> {'text/plain', headers}
+      case List.keytake(headers, ~c"content-type", 0) do
+        nil -> {~c"text/plain", headers}
         {{_, ct}, headers} -> {ct, headers}
       end
 
diff --git a/lib/tesla/middleware.ex b/lib/tesla/middleware.ex
index 660c26ae..4650d3e5 100644
--- a/lib/tesla/middleware.ex
+++ b/lib/tesla/middleware.ex
@@ -16,11 +16,11 @@ defmodule Tesla.Middleware do
       Tesla.client([{Tesla.Middleware.BaseUrl, "https://example.com"}])
       
   ## Ordering
-  
+
   The order in which middleware is defined matters. Note that the order when _sending_ the request
   matches the order the middleware was defined in, but the order when _receiving_ the response
   is reversed.
-  
+
   For example, `Tesla.Middleware.DecompressResponse` must come _after_ `Tesla.Middleware.JSON`,
   otherwise the response isn't decompressed before it reaches the JSON parser.
 
diff --git a/test/tesla/adapter/gun_test.exs b/test/tesla/adapter/gun_test.exs
index 924c18e0..4e61ad5f 100644
--- a/test/tesla/adapter/gun_test.exs
+++ b/test/tesla/adapter/gun_test.exs
@@ -197,7 +197,7 @@ defmodule Tesla.Adapter.GunTest do
     }
 
     assert {:error, "socks protocol is not supported"} ==
-             call(request, proxy: {:socks5, 'localhost', 1234})
+             call(request, proxy: {:socks5, ~c"localhost", 1234})
   end
 
   test "receive gun_up message when receive is false" do
diff --git a/test/tesla/middleware/timeout_test.exs b/test/tesla/middleware/timeout_test.exs
index 760ff573..3a51a76e 100644
--- a/test/tesla/middleware/timeout_test.exs
+++ b/test/tesla/middleware/timeout_test.exs
@@ -115,7 +115,7 @@ defmodule Tesla.Middleware.TimeoutTest do
           [{last_module, _, _, file_info} | _] = __STACKTRACE__
 
           assert Tesla.Middleware.TimeoutTest.Client == last_module
-          assert file_info[:file] == 'lib/tesla/builder.ex'
+          assert file_info[:file] == ~c"lib/tesla/builder.ex"
           assert file_info[:line] == 23
       else
         _ ->
@@ -131,7 +131,7 @@ defmodule Tesla.Middleware.TimeoutTest do
           [_, {timeout_module, _, _, module_file_info} | _] = __STACKTRACE__
 
           assert Tesla.Middleware.Timeout == timeout_module
-          assert module_file_info == [file: 'lib/tesla/middleware/timeout.ex', line: 59]
+          assert module_file_info == [file: ~c"lib/tesla/middleware/timeout.ex", line: 59]
       else
         _ ->
           flunk("Expected exception to be thrown")