incompatible data type in criteria expression error

I am doing a simple select by clicking on the month in the combobox it lists the people who work in this month
I have the month table (Num_Month"PK") and the person_month table (Num_Month"FK") both of integer type, and I checked well at execution I always get the error that data type incompatible in the criteria expression, I try with DBLookupComboBox too to import months from Month Table But the same error apear:

    With Q1 do
Begin
    Close;
    Sql.Clear;
    Sql.Add('Select * from Person_Month Where Num_month ='+QuotedStr(ComboBox1.Text));
    Open;
     DBGrid2.DataSource.DataSet.Open;
End;

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