Original User: mdenil
It seems that Huisletter sometimes contains a space character instead of an empty string.
A true empty string would not add space to your concatenated string
In your pre-logic block, test Huisletter for a single space
If it is a space set a variable to an empty string
if Huisletter is anything else, assume it is the actual Huisletter, and set the variable to the Huisletter value.
now build your concatenated string using the variable instead of Huisletter.