Combine fields in a formula where some fields are null

Hello,

I try to combine the fields Keteltype1, KetelType2 and KetelType3. The formula is correct, but I do not get the result I want.

Sometimes there is no data in the fields Keteltype1, Keteltype2 and Keteltype3. So these lines are blanc. The field Keteldeel is always filled with data.

This is the formula I use:

{@Keteldeel} + ' ' + {@KetelType1} + {@KetelType2} + {@KetelType3}

Can anyone let me know what the formula should be, so that I do not get blanc lines.

Thanks,

Tim