I'm basically trying to get a string variable and turn it into an integer but I'm getting this problem at line 46

Input string was not in a correct format.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.FormatException: Input string was not in a correct format.

Source Error:

dim pk as string
Line 44: pk = Session("pk")
Line 45:
Line 46: Dim pkInt As Integer = CType(pk, Integer)
Line 47:
Line 48: