reporting services - SSRS- Issue with iif containing monthname function -

I have a data set that I'm trying to force everything in the same group 12 months ago . I am trying to do this with a SQL case when it decides all months and years> 0 months ago 0 has an int value, but when I try to put it in SSR, it does not work.

I am trying to use this formula but do not work till I remove the 'Mahananam' function. = IIF (Fields ASGN_SUB_DUE_MONTH.Value> 0, MONTHNAME (Fields ASGN_SUB_DUE_MONTH.Value) & amp;! "" & Amp;! Fields ASGN_SUB_DUE_YEAR.Value
, "More than 12 months ago")

But this formula works = IIF (Fields ASGN_SUB_DUE_MONTH.Value> 0, MONTHNAME (IIF (Fields ASGN_SUB_DUE_MONTH.Value> 0 and Fields ASGN_SUB_DUE_MONTH.Value & lt; !! 13, Fields ASGN_SUB_DUE_MONTH.Value, 1)) & amp; ; "" & Fields! ASGN_SUB_DUE_YEAR.Value
, "More than 12 months ago")

Anyone know why and how I can solve The second function works, but I am not comfortable to leave it in reporting, which is included in the other functions of the embedded IEF.

In advance thank you

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The issue relates to my case statement and how iif in SSR Processes, true statements from my later research IIF because all the entries in the expression need to be embedded in all parts of the expression, even when I "month") that the IIF portion with MONTHNAME is not killed (values ​​of -1 or 0) Because of this evaluation and an error,