这是我在无法添加或更新子行时遇到的错误：
外键约束失败（selo.klijent，constraint klijent_ibfk_1 foreign key（gradd）references grad（gradd））
这是我的插入查询

string insertQuery = " INSERT INTO selo.Klijent(KlijentID,Ime,Prezime,Adresa,GradID,Telefon,Email,AktivanKlijent) Values('" + TB_Sifra + "','" + TB_Ime.Text + "','" + TB_Prezime.Text + "','" + TB_Adresa.Text + "','" + CB_Gradovi + "','" + TB_Telefon + "','" + TB_Mail.Text + "','" + proveraRB() + "')";

这是我的mysql代码

create table Klijent(

KlijentID INT NOT NULL AUTO_INCREMENT primary key,
Ime varchar(20) not null,
Prezime varchar(20) not null,
Adresa varchar(20) not null,
GradID  INT NOT NULL,
Telefon int not null,
Email varchar(20),
AktivanKlijent varchar(2),

FOREIGN KEY (GradID) REFERENCES Grad(GradID)
);

真的不知道该怎么办