Upgrading to FreeBSD 5.1

I am trying to upgrade my sysstem from 4.8-RELEASE-p4 to 5.1 

and i getting this error message when I run "make -j4 buildworld" 


[admin]/usr/src(101)#make -j4 buildworld
Running test variables
PASS: Test variables detected no regression, output matches.
Running test targets
PASS: Test targets detected no regression.
Running test sysvmatch
PASS: Test sysvmatch detected no regression.
Running test lhs_expn
FAIL: Test failed: regression detected.  See above.
*** Error code 1
1 error
*** Error code 2
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Building an up-to-date make(1)
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what do I need to check to fix this . I read the /src/UPDATING  ... I didnt find any info about this error message.


Thanx
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Didier