Variable Port Initialization - default values are not NULL

Variable Initialization 

The Integration Service does not set the initial value for variables to NULL. 
The Integration Service uses the following guidelines to set initial values for variables:

• Zero for numeric ports
• Empty strings for string ports
• 01/01/0001 for Date/Time ports

Therefore, use variables as counters, which need an initial value. For example, you can create a numeric variable with the following expression:
VAR1 + 1

This expression counts the number of rows in the VAR1 port. If the initial value of the variable were set to NULL, the expression would always evaluate to NULL. This is why the initial value is set to zero.