Trouble with my variable conversion

Question

Hi,
i am trying to accumulate a total to be stored in my 'targetTotal' variable of type int which is then to be sent to my msaccess db into a field called 'targetTotal' of type LongInteger, the trouble i am having now is that after adding my totals given the following code snippet:
targetOneDay = Convert.ToInt32(nine_to_fourteenhrstxbx.Text);//tonine_to_fourteenhrstxbx.Text;
targetTwoDay = Convert.ToInt32(fifteen_to_fortfourglhtxtbx.Text);
targetFiveDay = Convert.ToInt32(fourtyfive_to_74txtbx.Text);
targetTenDay = Convert.ToInt32(min75glhtxtbx.Text);
targetTotal = Convert.ToInt32(totaltargettxtbx.Text);
targetTotal = targetOneDay + targetTwoDay + targetFiveDay + targetTenDay;
i get an error which says the following: 'Input string was not in correct format' pointing to the target total variable. So i looked online for this particular error and based on my understanding i have to check if it is a null value and convert it accordingly. So what i first did was created another variable called 'total' of type int, removed the 'text' property of my target total and added the following if statement:
targetTotal = Convert.ToInt32(totaltargettxtbx);
if (targetTotal != null)
total = Convert.ToInt32(targetTotal);
else
total = 0;

now when i run my applicantion i get another error saying: Unable to cast object of type 'System.Windows.Forms.TextBox' to type 'System.IConvertible'. which is pointing to my targetTotal variable.
Can somebody kindly assist me with my issue please.