st: -svy: mean - problem with returned matrices

[Steven Samuels <ssamuels@albany.edu>]

Can anyone explain why I get the following error?  How can I access  
the elements of the matrix which generates the error?
Thanks,

Steve

Program:
/************************************/
webuse nhanes2, clear
svyset
svy : mean height
ereturn list
matrix list e(V_srs)
di el(e(V_srs),1,1)   //OKAY

matrix list e(b)
di el(e(b),1,1)      //GENERATES ERROR
/*************************************/

Log:
webuse nhanes2, clear

. svyset

      pweight: finalwgt
          VCE: linearized
     Strata 1: strata
         SU 1: psu
        FPC 1: <zero>

. svy : mean height
(running mean on estimation sample)

Survey: Mean estimation

Number of strata =      31          Number of obs    =   10351
Number of PSUs   =      62          Population size  = 1.2e+08
                                    Design df        =      31
--------------------------------------------------------------
             |             Linearized
             |       Mean   Std. Err.     [95% Conf. Interval]
-------------+------------------------------------------------
      height |   168.4599   .1466103      168.1609    168.7589
--------------------------------------------------------------

. ereturn list

scalars: [OUTPUT SKIPPED]
macros:  [OUTPUT SKIPPED]

matrices:
                 e(b) :  1 x 1
                 e(V) :  1 x 1
           e(_N_subp) :  1 x 1
             e(V_srs) :  1 x 1
                e(_N) :  1 x 1
             e(error) :  1 x 1

functions: [OUTPUT SKIPPED]

. matrix list e(V_srs)

symmetric e(V_srs)[1,1]
           height
height  .00908828

. di el(e(V_srs),1,1)   //OKAY
.00908828

. matrix list e(b)

symmetric e(b)[1,1]
       height
y1  168.45989

. di el(e(b),1,1)   //GENERATES ERROR
matrix operators that return matrices not allowed in this context
r(509);


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