diff --git a/errors_1_13_test.go b/errors_1_13_test.go
deleted file mode 100644
index eb56fe47..00000000
--- a/errors_1_13_test.go
+++ /dev/null
@@ -1,48 +0,0 @@
-// Copyright 2018 Twitch Interactive, Inc.  All Rights Reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License"). You may not
-// use this file except in compliance with the License. A copy of the License is
-// located at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// or in the "license" file accompanying this file. This file is distributed on
-// an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
-// express or implied. See the License for the specific language governing
-// permissions and limitations under the License.
-
-// +build go1.13
-
-package twirp_test
-
-import (
-	"errors"
-	"fmt"
-	"testing"
-
-	"github.com/twitchtv/twirp"
-)
-
-type myError string
-
-func (e myError) Error() string {
-	return string(e)
-}
-
-func TestInternalErrorWith_Unwrap(t *testing.T) {
-	myErr := myError("myError")
-	wrErr := fmt.Errorf("wrapped: %w", myErr) // double wrap
-	twerr := twirp.InternalErrorWith(wrErr)
-
-	if !errors.Is(twerr, myErr) {
-		t.Errorf("expected errors.Is to match the error wrapped by twirp.InternalErrorWith")
-	}
-
-	var errTarget myError
-	if !errors.As(twerr, &errTarget) {
-		t.Errorf("expected errors.As to match the error wrapped by twirp.InternalErrorWith")
-	}
-	if errTarget.Error() != myErr.Error() {
-		t.Errorf("invalid value for errTarget.Error(). have=%q, want=%q", errTarget.Error(), myErr.Error())
-	}
-}
diff --git a/errors_test.go b/errors_test.go
index 8711e08d..c77b5879 100644
--- a/errors_test.go
+++ b/errors_test.go
@@ -15,12 +15,13 @@ package twirp
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http/httptest"
 	"sync"
 	"testing"
 
-	"github.com/pkg/errors"
+	pkgerrors "github.com/pkg/errors"
 )
 
 func TestWithMetaRaces(t *testing.T) {
@@ -43,15 +44,53 @@ func TestWithMetaRaces(t *testing.T) {
 	}
 }
 
-func TestErrorCause(t *testing.T) {
-	rootCause := errors.New("this is only a test")
+func TestPkgErrorCause(t *testing.T) {
+	rootCause := pkgerrors.New("this is only a test")
 	twerr := InternalErrorWith(rootCause)
-	cause := errors.Cause(twerr)
+	cause := pkgerrors.Cause(twerr)
 	if cause != rootCause {
 		t.Errorf("got wrong cause for err. have=%q, want=%q", cause, rootCause)
 	}
 }
 
+func TestWrapError(t *testing.T) {
+	rootCause := errors.New("cause")
+	twerr := NewError(NotFound, "it ain't there")
+	err := WrapError(twerr, rootCause)
+	cause := pkgerrors.Cause(err)
+	if cause != rootCause {
+		t.Errorf("got wrong cause. got=%q, want=%q", cause, rootCause)
+	}
+	wantMsg := "twirp error not_found: it ain't there"
+	if gotMsg := err.Error(); gotMsg != wantMsg {
+		t.Errorf("got wrong error text. got=%q, want=%q", gotMsg, wantMsg)
+	}
+}
+
+type myError string
+
+func (e myError) Error() string {
+	return string(e)
+}
+
+func TestInternalErrorWith_Unwrap(t *testing.T) {
+	myErr := myError("myError")
+	wrErr := fmt.Errorf("wrapped: %w", myErr) // double wrap
+	twerr := InternalErrorWith(wrErr)
+
+	if !errors.Is(twerr, myErr) {
+		t.Errorf("expected errors.Is to match the error wrapped by twirp.InternalErrorWith")
+	}
+
+	var errTarget myError
+	if !errors.As(twerr, &errTarget) {
+		t.Errorf("expected errors.As to match the error wrapped by twirp.InternalErrorWith")
+	}
+	if errTarget.Error() != myErr.Error() {
+		t.Errorf("invalid value for errTarget.Error(). have=%q, want=%q", errTarget.Error(), myErr.Error())
+	}
+}
+
 type errorResponeWriter struct {
 	*httptest.ResponseRecorder
 }
@@ -132,17 +171,3 @@ func TestWriteError_WithNonTwirpError(t *testing.T) {
 		return
 	}
 }
-
-func TestWrapError(t *testing.T) {
-	rootCause := errors.New("cause")
-	twerr := NewError(NotFound, "it ain't there")
-	err := WrapError(twerr, rootCause)
-	cause := errors.Cause(err)
-	if cause != rootCause {
-		t.Errorf("got wrong cause. got=%q, want=%q", cause, rootCause)
-	}
-	wantMsg := "twirp error not_found: it ain't there"
-	if gotMsg := err.Error(); gotMsg != wantMsg {
-		t.Errorf("got wrong error text. got=%q, want=%q", gotMsg, wantMsg)
-	}
-}