diff --git a/core/mapping/unmarshaler.go b/core/mapping/unmarshaler.go
index 303805dd..b86ef8f4 100644
--- a/core/mapping/unmarshaler.go
+++ b/core/mapping/unmarshaler.go
@@ -271,6 +271,10 @@ func (u *Unmarshaler) processFieldPrimitiveWithJSONNumber(field reflect.StructFi
 			return err
 		}
 
+		if iValue < 0 {
+			return fmt.Errorf("unmarshal %q with bad value %q", fullName, v.String())
+		}
+
 		value.SetUint(uint64(iValue))
 	case reflect.Float32, reflect.Float64:
 		fValue, err := v.Float64()
@@ -727,10 +731,10 @@ func fillWithSameType(field reflect.StructField, value reflect.Value, mapValue i
 	if field.Type.Kind() == reflect.Ptr {
 		baseType := Deref(field.Type)
 		target := reflect.New(baseType).Elem()
-		target.Set(reflect.ValueOf(mapValue))
+		setSameKindValue(baseType, target, mapValue)
 		value.Set(target.Addr())
 	} else {
-		value.Set(reflect.ValueOf(mapValue))
+		setSameKindValue(field.Type, value, mapValue)
 	}
 
 	return nil
@@ -805,3 +809,11 @@ func readKeys(key string) []string {
 
 	return keys
 }
+
+func setSameKindValue(targetType reflect.Type, target reflect.Value, value interface{}) {
+	if reflect.ValueOf(value).Type().AssignableTo(targetType) {
+		target.Set(reflect.ValueOf(value))
+	} else {
+		target.Set(reflect.ValueOf(value).Convert(targetType))
+	}
+}
diff --git a/core/mapping/unmarshaler_test.go b/core/mapping/unmarshaler_test.go
index 1b3bdcfc..2cf21520 100644
--- a/core/mapping/unmarshaler_test.go
+++ b/core/mapping/unmarshaler_test.go
@@ -2660,6 +2660,86 @@ func TestUnmarshalJsonWithoutKey(t *testing.T) {
 	assert.Equal(t, "2", res.B)
 }
 
+func TestUnmarshalJsonUintNegative(t *testing.T) {
+	payload := `{"a": -1}`
+	var res struct {
+		A uint `json:"a"`
+	}
+	reader := strings.NewReader(payload)
+	err := UnmarshalJsonReader(reader, &res)
+	assert.NotNil(t, err)
+}
+
+func TestUnmarshalJsonDefinedInt(t *testing.T) {
+	type Int int
+	var res struct {
+		A Int `json:"a"`
+	}
+	payload := `{"a": -1}`
+	reader := strings.NewReader(payload)
+	err := UnmarshalJsonReader(reader, &res)
+	assert.Nil(t, err)
+	assert.Equal(t, Int(-1), res.A)
+}
+
+func TestUnmarshalJsonDefinedString(t *testing.T) {
+	type String string
+	var res struct {
+		A String `json:"a"`
+	}
+	payload := `{"a": "foo"}`
+	reader := strings.NewReader(payload)
+	err := UnmarshalJsonReader(reader, &res)
+	assert.Nil(t, err)
+	assert.Equal(t, String("foo"), res.A)
+}
+
+func TestUnmarshalJsonDefinedStringPtr(t *testing.T) {
+	type String string
+	var res struct {
+		A *String `json:"a"`
+	}
+	payload := `{"a": "foo"}`
+	reader := strings.NewReader(payload)
+	err := UnmarshalJsonReader(reader, &res)
+	assert.Nil(t, err)
+	assert.Equal(t, String("foo"), *res.A)
+}
+
+func TestUnmarshalJsonReaderComplex(t *testing.T) {
+	type (
+		MyInt      int
+		MyTxt      string
+		MyTxtArray []string
+
+		Req struct {
+			MyInt      MyInt      `json:"my_int"` // int.. ok
+			MyTxtArray MyTxtArray `json:"my_txt_array"`
+			MyTxt      MyTxt      `json:"my_txt"` // but string is not assignable
+			Int        int        `json:"int"`
+			Txt        string     `json:"txt"`
+		}
+	)
+	body := `{
+  "my_int": 100,
+  "my_txt_array": [
+    "a",
+    "b"
+  ],
+  "my_txt": "my_txt",
+  "int": 200,
+  "txt": "txt"
+}`
+	var req Req
+	err := UnmarshalJsonReader(strings.NewReader(body), &req)
+	assert.Nil(t, err)
+	assert.Equal(t, MyInt(100), req.MyInt)
+	assert.Equal(t, MyTxt("my_txt"), req.MyTxt)
+	assert.EqualValues(t, MyTxtArray([]string{"a", "b"}), req.MyTxtArray)
+	assert.Equal(t, 200, req.Int)
+	assert.Equal(t, "txt", req.Txt)
+}
+
 func BenchmarkDefaultValue(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		var a struct {