ORA-00932: inconsistent datatypes

hi, i hope that some can help me. i have an application on c#.net with ODP.NET (oracle data provider for .NET) but i get the following exception:

Oracle.DataAccess.Client.OracleException: ORA-00932: inconsistent datatypes

Line 83: cmd.ExecuteNonQuery();

the code of my application:
...
private void Button1_Click(object sender, System.EventArgs e)
{
DateTime datum = new DateTime();
datum = DateTime.Now;

string ConStr = "User Id=user1;Password=bllablla;Data Source=localhost";
OracleConnection con = new OracleConnection(ConStr);
con.Open();
OracleCommand cmd = con.CreateCommand();
cmd.CommandText = "insert into artikel(benname,gebiet,datum,text,gelesen) values ('ruki', : pGebiet, : pDate, : pText,0 )";
cmd.CommandType = CommandType.Text;

OracleParameter prm = new OracleParameter("pDate",OracleDbType.Date);
OracleParameter prm1 = new OracleParameter("pText",OracleDbType.Clob);
OracleParameter prm2 = new OracleParameter("pGebiet",OracleDbType.Varchar2);

prm.Direction = ParameterDirection.Input;
prm1.Direction = ParameterDirection.Input;
prm2.Direction = ParameterDirection.Input;
//Oracle.DataAccess.Types.OracleString vGebiet = new Oracle.DataAccess.Types.OracleString(TextBox2.Text);

//vGebiet = TextBox2.Text;
prm.Value = datum; // it works
prm1.Value = TextBox1.Text; // it works
prm2.Value = TextBox2.Text; // it doesn't work

cmd.Parameters.Add(prm);
cmd.Parameters.Add(prm1);
cmd.Parameters.Add(prm2);

cmd.ExecuteNonQuery();
}

my oracle table is :

create table artikel (
benname varchar2(15),
gebiet varchar2(30),
datum date,
text clob not null,
gelesen number,
primary key (benname,datum) );

i hope that somebody can help. thanks

I don't know about the ADO.NET, but as long as your concern with Oracle, this error message occured when you try to pass a wrong data type value into a wrong data type that is not implicitly convertable, for example if you try to pass any numeric value into a character, an automatic conversion is applied and you get no error, but if you try to enter character value into a numeric field or numeric/character value into date, implicit conversion does not apply and you get error message.

yes, i think im giving to a varchar2 a string and i think that's ok? isn't it?

Describe ARTIKEL
The datatype for GEBIET must be VARCHAR2. It may not be.

You hit error this most often on fields with the LONG datatype.

One other obscure cause: you are trying to access a system table that normally is not accesible - usually because of a misspelling