Conditional split error for a field

I have a field with values 

Column
--------------------
4.3.3
OEM DocAve HF #1
Corp STS Cert Update
Corp STS Cert Update
2.0.0.1
1.0.0.1

(ISNULL([Column]) ? (DT_WSTR,50)(" ") : (DT_WSTR,50)([Column])) != (ISNULL([Column_Dest]) ? (DT_WSTR,50)(" ") : (DT_WSTR,50)([Column_Dest]))

This expression gives me error, am i missing anything in this expression?