fix get attachment lost nil key
diff --git a/request.go b/request.go
index 807f73f..9518045 100644
--- a/request.go
+++ b/request.go
@@ -23,9 +23,7 @@
 	"strconv"
 	"strings"
 	"time"
-)
 
-import (
 	perrors "github.com/pkg/errors"
 )
 
@@ -337,6 +335,11 @@
 	dest := make(map[string]string)
 	for k, v := range origin {
 		if kv, ok := k.(string); ok {
+			if v == nil {
+				dest[kv] = ""
+				continue
+			}
+
 			if vv, ok := v.(string); ok {
 				dest[kv] = vv
 			}