do not clear error formatting informative field

entry.go

@@ -108,7 +108,7 @@ func (entry *Entry) WithFields(fields Fields) *Entry {
 	for k, v := range entry.Data {
 		data[k] = v
 	}
-	var fieldErr string
+	fieldErr := entry.err
 	for k, v := range fields {
 		isErrField := false
 		if t := reflect.TypeOf(v); t != nil {
@@ -120,9 +120,11 @@ func (entry *Entry) WithFields(fields Fields) *Entry {
 			}
 		}
 		if isErrField {
-			fieldErr = fmt.Sprintf("can not add field %q", k)
-			if entry.err != "" {
-				fieldErr = entry.err + ", " + fieldErr
+			tmp := fmt.Sprintf("can not add field %q", k)
+			if fieldErr != "" {
+				fieldErr = entry.err + ", " + tmp
+			} else {
+				fieldErr = tmp
 			}
 		} else {
 			data[k] = v
@@ -133,7 +135,7 @@ func (entry *Entry) WithFields(fields Fields) *Entry {
 
 // Overrides the time of the Entry.
 func (entry *Entry) WithTime(t time.Time) *Entry {
-	return &Entry{Logger: entry.Logger, Data: entry.Data, Time: t}
+	return &Entry{Logger: entry.Logger, Data: entry.Data, Time: t, err: entry.err}
 }
 
 // getPackageName reduces a fully qualified function name to the package name

entry_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -125,4 +126,16 @@ func TestEntryWithIncorrectField(t *testing.T) {
 
 	assert.Equal(eWithFunc.err, `can not add field "func"`)
 	assert.Equal(eWithFuncPtr.err, `can not add field "funcPtr"`)
+
+	eWithFunc = eWithFunc.WithField("not_a_func", "it is a string")
+	eWithFuncPtr = eWithFuncPtr.WithField("not_a_func", "it is a string")
+
+	assert.Equal(eWithFunc.err, `can not add field "func"`)
+	assert.Equal(eWithFuncPtr.err, `can not add field "funcPtr"`)
+
+	eWithFunc = eWithFunc.WithTime(time.Now())
+	eWithFuncPtr = eWithFuncPtr.WithTime(time.Now())
+
+	assert.Equal(eWithFunc.err, `can not add field "func"`)
+	assert.Equal(eWithFuncPtr.err, `can not add field "funcPtr"`)
 }