fixes #88

json_formatter.go

@@ -9,12 +9,16 @@ import (
 type JSONFormatter struct{}
 
 func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
-	prefixFieldClashes(entry)
-	entry.Data["time"] = entry.Time.Format(time.RFC3339)
-	entry.Data["msg"] = entry.Message
-	entry.Data["level"] = entry.Level.String()
+	data := make(Fields, len(entry.Data)+3)
+	for k, v := range entry.Data {
+		data[k] = v
+	}
+	prefixFieldClashes(data)
+	data["time"] = entry.Time.Format(time.RFC3339)
+	data["msg"] = entry.Message
+	data["level"] = entry.Level.String()
 
-	serialized, err := json.Marshal(entry.Data)
+	serialized, err := json.Marshal(data)
 	if err != nil {
 		return nil, fmt.Errorf("Failed to marshal fields to JSON, %v", err)
 	}