Combine two columns of same data type into single column

Hi,
I have a table with the following data:

TABLE TAB1

ID FIRST_NAME LAST_NAME GROUP
-------- ---------- ---------- ----------
1 SMITH D B2
2 ALLEN G A4
3 BAKER J H9

I want to see the result data as follows:

FULL_NAME
--------------
SMITH D

I tried to get result using "select (FIRST_NAME+LAST_NAME) as FULL_NAME from tab" and also "select FIRST_NAME+LAST_NAME as FULL_NAME from tab"

The error i got is
SQL0420N Invalid character found in a character string argument of the
function "DECFLOAT". SQLSTATE=22018 :o

Please help me to achieve this.