diff --git a/niljson_test.go b/niljson_test.go
index 315029c..c3f3f3d 100644
--- a/niljson_test.go
+++ b/niljson_test.go
@@ -12,6 +12,13 @@ import (
 	"testing"
 )
 
+const (
+	ErrUnmarshalFailed    = "failed to unmarshal json with nil types: %s"
+	ErrMarshalFailed      = "failed to marshal json with nil types: %s"
+	ErrExpectedJSONString = "expected json to be %q, got %q"
+	ErrExpectedJSONInt    = "expected json to be %d, got %d"
+)
+
 var jsonBytes = []byte(
 	`{
 		"bool": true,
@@ -37,7 +44,7 @@ func TestVariable_UnmarshalJSON_Boolean(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -68,10 +75,10 @@ func TestVariable_MarshalJSON_Boolean(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -83,7 +90,7 @@ func TestVariable_UnmarshalJSON_ByteSlice(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -114,10 +121,10 @@ func TestVariable_MarshalJSON_ByteSlice(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -130,7 +137,7 @@ func TestVariable_UnmarshalJSON_Float32(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -161,10 +168,10 @@ func TestVariable_MarshalJSON_Float32(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -177,7 +184,7 @@ func TestVariable_UnmarshalJSON_Float64(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -208,10 +215,10 @@ func TestVariable_MarshalJSON_Float64(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -224,7 +231,7 @@ func TestVariable_UnmarshalJSON_Int(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -234,7 +241,7 @@ func TestVariable_UnmarshalJSON_Int(t *testing.T) {
 		t.Errorf("expected nil, but got value")
 	}
 	if jt.Value.Value() != expected {
-		t.Errorf("expected value to be %d, got %d", expected, jt.Value.Value())
+		t.Errorf(ErrExpectedJSONInt, expected, jt.Value.Value())
 	}
 
 	jt.Value.Reset()
@@ -255,10 +262,10 @@ func TestVariable_MarshalJSON_Int(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -271,7 +278,7 @@ func TestVariable_UnmarshalJSON_Int64(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -281,7 +288,7 @@ func TestVariable_UnmarshalJSON_Int64(t *testing.T) {
 		t.Errorf("expected nil, but got value")
 	}
 	if jt.Value.Value() != expected {
-		t.Errorf("expected value to be %d, got %d", expected, jt.Value.Value())
+		t.Errorf(ErrExpectedJSONInt, expected, jt.Value.Value())
 	}
 
 	jt.Value.Reset()
@@ -302,10 +309,10 @@ func TestVariable_MarshalJSON_Int64(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -318,7 +325,7 @@ func TestVariable_UnmarshalJSON_String(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -349,10 +356,10 @@ func TestVariable_MarshalJSON_String(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -365,7 +372,7 @@ func TestVariable_UnmarshalJSON_UInt(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -375,7 +382,7 @@ func TestVariable_UnmarshalJSON_UInt(t *testing.T) {
 		t.Errorf("expected nil, but got value")
 	}
 	if jt.Value.Value() != expected {
-		t.Errorf("expected value to be %d, got %d", expected, jt.Value.Value())
+		t.Errorf(ErrExpectedJSONInt, expected, jt.Value.Value())
 	}
 
 	jt.Value.Reset()
@@ -396,10 +403,10 @@ func TestVariable_MarshalJSON_UInt(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -412,7 +419,7 @@ func TestVariable_UnmarshalJSON_UInt8(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -422,7 +429,7 @@ func TestVariable_UnmarshalJSON_UInt8(t *testing.T) {
 		t.Errorf("expected nil, but got value")
 	}
 	if jt.Value.Value() != expected {
-		t.Errorf("expected value to be %d, got %d", expected, jt.Value.Value())
+		t.Errorf(ErrExpectedJSONInt, expected, jt.Value.Value())
 	}
 
 	jt.Value.Reset()
@@ -443,10 +450,10 @@ func TestVariable_MarshalJSON_UInt8(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -459,7 +466,7 @@ func TestVariable_UnmarshalJSON_UInt16(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -469,7 +476,7 @@ func TestVariable_UnmarshalJSON_UInt16(t *testing.T) {
 		t.Errorf("expected nil, but got value")
 	}
 	if jt.Value.Value() != expected {
-		t.Errorf("expected value to be %d, got %d", expected, jt.Value.Value())
+		t.Errorf(ErrExpectedJSONInt, expected, jt.Value.Value())
 	}
 
 	jt.Value.Reset()
@@ -490,10 +497,10 @@ func TestVariable_MarshalJSON_UInt16(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -506,7 +513,7 @@ func TestVariable_UnmarshalJSON_UInt32(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -516,7 +523,7 @@ func TestVariable_UnmarshalJSON_UInt32(t *testing.T) {
 		t.Errorf("expected nil, but got value")
 	}
 	if jt.Value.Value() != expected {
-		t.Errorf("expected value to be %d, got %d", expected, jt.Value.Value())
+		t.Errorf(ErrExpectedJSONInt, expected, jt.Value.Value())
 	}
 
 	jt.Value.Reset()
@@ -537,10 +544,10 @@ func TestVariable_MarshalJSON_UInt32(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }
 
@@ -553,7 +560,7 @@ func TestVariable_UnmarshalJSON_UInt64(t *testing.T) {
 
 	var jt JSONType
 	if err := json.Unmarshal(jsonBytes, &jt); err != nil {
-		t.Errorf("failed to unmarshal json with nil types: %s", err)
+		t.Errorf(ErrUnmarshalFailed, err)
 	}
 
 	if jt.Value.IsNil() {
@@ -563,7 +570,7 @@ func TestVariable_UnmarshalJSON_UInt64(t *testing.T) {
 		t.Errorf("expected nil, but got value")
 	}
 	if jt.Value.Value() != expected {
-		t.Errorf("expected value to be %d, got %d", expected, jt.Value.Value())
+		t.Errorf(ErrExpectedJSONInt, expected, jt.Value.Value())
 	}
 
 	jt.Value.Reset()
@@ -584,10 +591,10 @@ func TestVariable_MarshalJSON_UInt64(t *testing.T) {
 	}
 	data, err := json.Marshal(&jt)
 	if err != nil {
-		t.Errorf("failed to marshal json with nil types: %s", err)
+		t.Errorf(ErrMarshalFailed, err)
 	}
 	if !bytes.Equal(data, []byte(expected)) {
-		t.Errorf("expected json to be %q, got %q", expected, string(data))
+		t.Errorf(ErrExpectedJSONString, expected, string(data))
 	}
 }