Cannot cast the error output on a SSIS insert task

We are doing a SSIS package to do some data transfer and insert it in a table (SQL SERVER 2008). It fails for one of the row and we want to log the issue by redirecting the error output. So far so good. The problem is it won't let us cast one of the column, a DT_I4, to a string (DT_STR or开发者_JS百科 DT_WSTR). We get this error everytime :

 [Log History Write Error [338]] Error: SSIS Error Code DTS_E_INDUCEDTRANSFORMFAILUREONERROR

The funny part is, if we change our destination element to a fast-load, we are actually able to cast the error output to a string. Would anybody know why is that?