diff --git a/binding/form_mapping.go b/binding/form_mapping.go
index 2f4e45b40f..421c0f7124 100644
--- a/binding/form_mapping.go
+++ b/binding/form_mapping.go
@@ -16,7 +16,15 @@ import (
 	"github.com/gin-gonic/gin/internal/json"
 )
 
-var errUnknownType = errors.New("unknown type")
+var (
+	errUnknownType = errors.New("unknown type")
+
+	// ErrConvertMapStringSlice can not covert to map[string][]string
+	ErrConvertMapStringSlice = errors.New("can not convert to map slices of strings")
+
+	// ErrConvertToMapString can not convert to map[string]string
+	ErrConvertToMapString = errors.New("can not convert to map of strings")
+)
 
 func mapUri(ptr interface{}, m map[string][]string) error {
 	return mapFormByTag(ptr, m, "uri")
@@ -109,7 +117,7 @@ func mapping(value reflect.Value, field reflect.StructField, setter setter, tag
 			if sf.PkgPath != "" && !sf.Anonymous { // unexported
 				continue
 			}
-			ok, err := mapping(value.Field(i), tValue.Field(i), setter, tag)
+			ok, err := mapping(value.Field(i), sf, setter, tag)
 			if err != nil {
 				return false, err
 			}
@@ -371,7 +379,7 @@ func setFormMap(ptr interface{}, form map[string][]string) error {
 	if el.Kind() == reflect.Slice {
 		ptrMap, ok := ptr.(map[string][]string)
 		if !ok {
-			return errors.New("cannot convert to map slices of strings")
+			return ErrConvertMapStringSlice
 		}
 		for k, v := range form {
 			ptrMap[k] = v
@@ -382,7 +390,7 @@ func setFormMap(ptr interface{}, form map[string][]string) error {
 
 	ptrMap, ok := ptr.(map[string]string)
 	if !ok {
-		return errors.New("cannot convert to map of strings")
+		return ErrConvertToMapString
 	}
 	for k, v := range form {
 		ptrMap[k] = v[len(v)-1] // pick last
diff --git a/binding/header.go b/binding/header.go
index 179ce4ea20..b99302af82 100644
--- a/binding/header.go
+++ b/binding/header.go
@@ -29,6 +29,6 @@ type headerSource map[string][]string
 
 var _ setter = headerSource(nil)
 
-func (hs headerSource) TrySet(value reflect.Value, field reflect.StructField, tagValue string, opt setOptions) (isSetted bool, err error) {
+func (hs headerSource) TrySet(value reflect.Value, field reflect.StructField, tagValue string, opt setOptions) (bool, error) {
 	return setByForm(value, field, hs, textproto.CanonicalMIMEHeaderKey(tagValue), opt)
 }
diff --git a/binding/multipart_form_mapping.go b/binding/multipart_form_mapping.go
index f85a1aa604..69c0a5443d 100644
--- a/binding/multipart_form_mapping.go
+++ b/binding/multipart_form_mapping.go
@@ -15,8 +15,16 @@ type multipartRequest http.Request
 
 var _ setter = (*multipartRequest)(nil)
 
+var (
+	// ErrMultiFileHeader multipart.FileHeader invalid
+	ErrMultiFileHeader = errors.New("unsupported field type for multipart.FileHeader")
+
+	// ErrMultiFileHeaderLenInvalid array for []*multipart.FileHeader len invalid
+	ErrMultiFileHeaderLenInvalid = errors.New("unsupported len of array for []*multipart.FileHeader")
+)
+
 // TrySet tries to set a value by the multipart request with the binding a form file
-func (r *multipartRequest) TrySet(value reflect.Value, field reflect.StructField, key string, opt setOptions) (isSetted bool, err error) {
+func (r *multipartRequest) TrySet(value reflect.Value, field reflect.StructField, key string, opt setOptions) (bool, error) {
 	if files := r.MultipartForm.File[key]; len(files) != 0 {
 		return setByMultipartFormFile(value, field, files)
 	}
@@ -49,12 +57,12 @@ func setByMultipartFormFile(value reflect.Value, field reflect.StructField, file
 	case reflect.Array:
 		return setArrayOfMultipartFormFiles(value, field, files)
 	}
-	return false, errors.New("unsupported field type for multipart.FileHeader")
+	return false, ErrMultiFileHeader
 }
 
 func setArrayOfMultipartFormFiles(value reflect.Value, field reflect.StructField, files []*multipart.FileHeader) (isSetted bool, err error) {
 	if value.Len() != len(files) {
-		return false, errors.New("unsupported len of array for []*multipart.FileHeader")
+		return false, ErrMultiFileHeaderLenInvalid
 	}
 	for i := range files {
 		setted, err := setByMultipartFormFile(value.Index(i), field, files[i:i+1])
diff --git a/render/json.go b/render/json.go
index 4186309398..e25415b001 100644
--- a/render/json.go
+++ b/render/json.go
@@ -46,9 +46,11 @@ type PureJSON struct {
 	Data interface{}
 }
 
-var jsonContentType = []string{"application/json; charset=utf-8"}
-var jsonpContentType = []string{"application/javascript; charset=utf-8"}
-var jsonAsciiContentType = []string{"application/json"}
+var (
+	jsonContentType      = []string{"application/json; charset=utf-8"}
+	jsonpContentType     = []string{"application/javascript; charset=utf-8"}
+	jsonAsciiContentType = []string{"application/json"}
+)
 
 // Render (JSON) writes data with custom ContentType.
 func (r JSON) Render(w http.ResponseWriter) (err error) {