diff --git a/internal/gperr/builder.go b/internal/gperr/builder.go
index 68922f0..21eca3a 100644
--- a/internal/gperr/builder.go
+++ b/internal/gperr/builder.go
@@ -59,6 +59,9 @@ func (b *Builder) Error() Error {
 	if len(b.errs) == 0 {
 		return nil
 	}
+	if len(b.errs) == 1 && b.about == "" {
+		return wrap(b.errs[0])
+	}
 	return &nestedError{Err: New(b.about), Extras: b.errs}
 }
 
diff --git a/internal/gperr/nested_error.go b/internal/gperr/nested_error.go
index 7b620c3..888f350 100644
--- a/internal/gperr/nested_error.go
+++ b/internal/gperr/nested_error.go
@@ -11,9 +11,11 @@ type nestedError struct {
 	Extras []error `json:"extras"`
 }
 
+var emptyError = errStr("")
+
 func (err nestedError) Subject(subject string) Error {
 	if err.Err == nil {
-		err.Err = PrependSubject(subject, errStr(""))
+		err.Err = PrependSubject(subject, emptyError)
 	} else {
 		err.Err = PrependSubject(subject, err.Err)
 	}
diff --git a/internal/gperr/subject.go b/internal/gperr/subject.go
index bed2b72..3a336f0 100644
--- a/internal/gperr/subject.go
+++ b/internal/gperr/subject.go
@@ -45,6 +45,11 @@ func PrependSubject(subject string, err error) error {
 	switch err := err.(type) {
 	case *withSubject:
 		return err.Prepend(subject)
+	case *wrappedError:
+		return &wrappedError{
+			Err:     PrependSubject(subject, err.Err),
+			Message: err.Message,
+		}
 	case Error:
 		return err.Subject(subject)
 	}
@@ -95,20 +100,24 @@ func (err *withSubject) Markdown() []byte {
 
 func (err *withSubject) fmtError(highlight highlightFunc) []byte {
 	// subject is in reversed order
-	n := len(err.Subjects)
 	size := 0
 	errStr := err.Err.Error()
+	subjects := err.Subjects
+	if err.pendingSubject != "" {
+		subjects = append(subjects, err.pendingSubject)
+	}
 	var buf bytes.Buffer
-	for _, s := range err.Subjects {
+	for _, s := range subjects {
 		size += len(s)
 	}
+	n := len(subjects)
 	buf.Grow(size + 2 + n*len(subjectSep) + len(errStr) + len(highlight("")))
 
 	for i := n - 1; i > 0; i-- {
-		buf.WriteString(err.Subjects[i])
+		buf.WriteString(subjects[i])
 		buf.WriteString(subjectSep)
 	}
-	buf.WriteString(highlight(err.Subjects[0]))
+	buf.WriteString(highlight(subjects[0]))
 	if errStr != "" {
 		buf.WriteString(": ")
 		buf.WriteString(errStr)
diff --git a/internal/utils/serialization.go b/internal/utils/serialization.go
index cecb193..b2930d5 100644
--- a/internal/utils/serialization.go
+++ b/internal/utils/serialization.go
@@ -256,7 +256,7 @@ func mapUnmarshalValidate(src SerializedObject, dst any, checkValidateTag bool)
 			if field, ok := mapping[strutils.ToLowerNoSnake(k)]; ok {
 				err := Convert(reflect.ValueOf(v), field, !hasValidateTag)
 				if err != nil {
-					errs.Add(err)
+					errs.Add(err.Subject(k))
 				}
 			} else {
 				errs.Add(ErrUnknownField.Subject(k).With(gperr.DoYouMean(NearestField(k, mapping))))
@@ -330,10 +330,6 @@ func Convert(src reflect.Value, dst reflect.Value, checkValidateTag bool) gperr.
 		srcT = src.Type()
 	}
 
-	if !dst.CanSet() {
-		return ErrUnsettable.Subject(dstT.String())
-	}
-
 	if dst.Kind() == reflect.Pointer {
 		if dst.IsNil() {
 			dst.Set(New(dstT.Elem()))
@@ -346,16 +342,25 @@ func Convert(src reflect.Value, dst reflect.Value, checkValidateTag bool) gperr.
 
 	switch {
 	case srcT.AssignableTo(dstT):
+		if !dst.CanSet() {
+			return ErrUnsettable.Subject(dstT.String())
+		}
 		dst.Set(src)
 		return nil
 	// case srcT.ConvertibleTo(dstT):
 	// 	dst.Set(src.Convert(dstT))
 	// 	return nil
 	case srcKind == reflect.String:
+		if !dst.CanSet() {
+			return ErrUnsettable.Subject(dstT.String())
+		}
 		if convertible, err := ConvertString(src.String(), dst); convertible {
 			return err
 		}
 	case isIntFloat(srcKind):
+		if !dst.CanSet() {
+			return ErrUnsettable.Subject(dstT.String())
+		}
 		var strV string
 		switch {
 		case src.CanInt():
@@ -386,7 +391,7 @@ func Convert(src reflect.Value, dst reflect.Value, checkValidateTag bool) gperr.
 		if dstT.Kind() != reflect.Slice {
 			return ErrUnsupportedConversion.Subject(dstT.String() + " to " + srcT.String())
 		}
-		sliceErrs := gperr.NewBuilder("slice conversion errors")
+		sliceErrs := gperr.NewBuilder()
 		newSlice := reflect.MakeSlice(dstT, src.Len(), src.Len())
 		i := 0
 		for j, v := range src.Seq2() {
@@ -469,7 +474,7 @@ func ConvertString(src string, dst reflect.Value) (convertible bool, convErr gpe
 		if !isMultiline && src[0] != '-' {
 			values := strutils.CommaSeperatedList(src)
 			dst.Set(reflect.MakeSlice(dst.Type(), len(values), len(values)))
-			errs := gperr.NewBuilder("invalid slice values")
+			errs := gperr.NewBuilder()
 			for i, v := range values {
 				err := Convert(reflect.ValueOf(v), dst.Index(i), true)
 				if err != nil {