I am attempting to calculate average assets i.e. computed as average assets = (assets (t) + assets (t-1) ) / 2
I have used this code to generate average assets:

by fid: gen avg_assets = (assets + assets[_n-1]) / 2

The computed value of average assets looks like as follows. The problem is that ever when the previous year (t-1) value of asset is represented as empty cell represented as "." the STATA treats it as 0 and gives me the wrong estimation of average assets. In reality, if one of the assets value is unavailable as represented by ".", STATA must return average asset value as ".", instead of treating it as zero. Please help.

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input str18 firmid int time str22 firmid_time float(fid assets_new avg_assets)
"0876711D LN Equity" 2003 "0876711D LN Equity2003" 1       .         .
"0876711D LN Equity" 2004 "0876711D LN Equity2004" 1       .         0
"0876711D LN Equity" 2005 "0876711D LN Equity2005" 1       .         0
"0876711D LN Equity" 2006 "0876711D LN Equity2006" 1       .         0
"0876711D LN Equity" 2007 "0876711D LN Equity2007" 1       .         0
"0876711D LN Equity" 2008 "0876711D LN Equity2008" 1   830.7    415.35
"0876711D LN Equity" 2009 "0876711D LN Equity2009" 1   714.2    772.45
"0876711D LN Equity" 2010 "0876711D LN Equity2010" 1   788.6     751.4
"0876711D LN Equity" 2011 "0876711D LN Equity2011" 1     727     757.8
"0876711D LN Equity" 2012 "0876711D LN Equity2012" 1   684.7    705.85
"0876711D LN Equity" 2013 "0876711D LN Equity2013" 1       .    342.35
"0876711D LN Equity" 2014 "0876711D LN Equity2014" 1       .         0
"0876711D LN Equity" 2015 "0876711D LN Equity2015" 1       .         0
"0876711D LN Equity" 2016 "0876711D LN Equity2016" 1       .         0
"0876711D LN Equity" 2017 "0876711D LN Equity2017" 1       .         0
"0876711D LN Equity" 2018 "0876711D LN Equity2018" 1       .         0
"1218069D LN Equity" 2003 "1218069D LN Equity2003" 2 182.889         .
"1218069D LN Equity" 2004 "1218069D LN Equity2004" 2  232.52  207.7045
"1218069D LN Equity" 2005 "1218069D LN Equity2005" 2 275.615  254.0675
"1218069D LN Equity" 2006 "1218069D LN Equity2006" 2 717.403   496.509
"1218069D LN Equity" 2007 "1218069D LN Equity2007" 2 790.272  753.8375
"1218069D LN Equity" 2008 "1218069D LN Equity2008" 2 903.164   846.718
"1218069D LN Equity" 2009 "1218069D LN Equity2009" 2 937.341  920.2525
"1218069D LN Equity" 2010 "1218069D LN Equity2010" 2 910.465   923.903
"1218069D LN Equity" 2011 "1218069D LN Equity2011" 2 823.901   867.183
"1218069D LN Equity" 2012 "1218069D LN Equity2012" 2       .  411.9505
"1218069D LN Equity" 2013 "1218069D LN Equity2013" 2       .         0
"1218069D LN Equity" 2014 "1218069D LN Equity2014" 2       .         0
"1218069D LN Equity" 2015 "1218069D LN Equity2015" 2       .         0
"1218069D LN Equity" 2016 "1218069D LN Equity2016" 2       .         0
"1218069D LN Equity" 2017 "1218069D LN Equity2017" 2       .         0
"1218069D LN Equity" 2018 "1218069D LN Equity2018" 2       .         0
"1334987D LN Equity" 2003 "1334987D LN Equity2003" 3 126.402         .
"1334987D LN Equity" 2004 "1334987D LN Equity2004" 3 191.404   158.903
"1334987D LN Equity" 2005 "1334987D LN Equity2005" 3  216.22   203.812
"1334987D LN Equity" 2006 "1334987D LN Equity2006" 3 279.354   247.787
"1334987D LN Equity" 2007 "1334987D LN Equity2007" 3 260.482   269.918
"1334987D LN Equity" 2008 "1334987D LN Equity2008" 3   212.8   236.641
"1334987D LN Equity" 2009 "1334987D LN Equity2009" 3   245.4     229.1
"1334987D LN Equity" 2010 "1334987D LN Equity2010" 3   223.2     234.3
"1334987D LN Equity" 2011 "1334987D LN Equity2011" 3       .     111.6
"1334987D LN Equity" 2012 "1334987D LN Equity2012" 3       .         0
"1334987D LN Equity" 2013 "1334987D LN Equity2013" 3       .         0
"1334987D LN Equity" 2014 "1334987D LN Equity2014" 3       .         0
"1334987D LN Equity" 2015 "1334987D LN Equity2015" 3       .         0
"1334987D LN Equity" 2016 "1334987D LN Equity2016" 3       .         0
"1334987D LN Equity" 2017 "1334987D LN Equity2017" 3       .         0
"1334987D LN Equity" 2018 "1334987D LN Equity2018" 3       .         0
"1561649D LN Equity" 2003 "1561649D LN Equity2003" 4 205.089         .
"1561649D LN Equity" 2004 "1561649D LN Equity2004" 4 212.072  208.5805
"1561649D LN Equity" 2005 "1561649D LN Equity2005" 4 203.671  207.8715
"1561649D LN Equity" 2006 "1561649D LN Equity2006" 4 247.038  225.3545
"1561649D LN Equity" 2007 "1561649D LN Equity2007" 4       .   123.519
"1561649D LN Equity" 2008 "1561649D LN Equity2008" 4 690.709  345.3545
"1561649D LN Equity" 2009 "1561649D LN Equity2009" 4  830.02  760.3645
"1561649D LN Equity" 2010 "1561649D LN Equity2010" 4  1565.9   1197.96
"1561649D LN Equity" 2011 "1561649D LN Equity2011" 4  1343.3    1454.6
"1561649D LN Equity" 2012 "1561649D LN Equity2012" 4  1488.1    1415.7
"1561649D LN Equity" 2013 "1561649D LN Equity2013" 4  1271.1    1379.6
"1561649D LN Equity" 2014 "1561649D LN Equity2014" 4  2122.2   1696.65
"1561649D LN Equity" 2015 "1561649D LN Equity2015" 4       .    1061.1
"1561649D LN Equity" 2016 "1561649D LN Equity2016" 4       .         0
"1561649D LN Equity" 2017 "1561649D LN Equity2017" 4       .         0
"1561649D LN Equity" 2018 "1561649D LN Equity2018" 4       .         0
"1638414D LN Equity" 2003 "1638414D LN Equity2003" 5 145.775         .
"1638414D LN Equity" 2004 "1638414D LN Equity2004" 5 145.237   145.506
"1638414D LN Equity" 2005 "1638414D LN Equity2005" 5  149.61  147.4235
"1638414D LN Equity" 2006 "1638414D LN Equity2006" 5 221.261  185.4355
"1638414D LN Equity" 2007 "1638414D LN Equity2007" 5 207.725   214.493
"1638414D LN Equity" 2008 "1638414D LN Equity2008" 5 177.811   192.768
"1638414D LN Equity" 2009 "1638414D LN Equity2009" 5 163.834  170.8225
"1638414D LN Equity" 2010 "1638414D LN Equity2010" 5 164.699  164.2665
"1638414D LN Equity" 2011 "1638414D LN Equity2011" 5 145.875   155.287
"1638414D LN Equity" 2012 "1638414D LN Equity2012" 5 193.526  169.7005
"1638414D LN Equity" 2013 "1638414D LN Equity2013" 5 187.314    190.42
"1638414D LN Equity" 2014 "1638414D LN Equity2014" 5 209.428   198.371
"1638414D LN Equity" 2015 "1638414D LN Equity2015" 5       .   104.714
"1638414D LN Equity" 2016 "1638414D LN Equity2016" 5       .         0
"1638414D LN Equity" 2017 "1638414D LN Equity2017" 5       .         0
"1638414D LN Equity" 2018 "1638414D LN Equity2018" 5       .         0
"1655637D LN Equity" 2003 "1655637D LN Equity2003" 6  1951.4         .
"1655637D LN Equity" 2004 "1655637D LN Equity2004" 6  2336.2    2143.8
"1655637D LN Equity" 2005 "1655637D LN Equity2005" 6  3607.2    2971.7
"1655637D LN Equity" 2006 "1655637D LN Equity2006" 6  3446.8      3527
"1655637D LN Equity" 2007 "1655637D LN Equity2007" 6  3579.5   3513.15
"1655637D LN Equity" 2008 "1655637D LN Equity2008" 6  4117.8   3848.65
"1655637D LN Equity" 2009 "1655637D LN Equity2009" 6    5673    4895.4
"1655637D LN Equity" 2010 "1655637D LN Equity2010" 6  6114.6    5893.8
"1655637D LN Equity" 2011 "1655637D LN Equity2011" 6  6502.1   6308.35
"1655637D LN Equity" 2012 "1655637D LN Equity2012" 6  6514.9    6508.5
"1655637D LN Equity" 2013 "1655637D LN Equity2013" 6  6573.1      6544
"1655637D LN Equity" 2014 "1655637D LN Equity2014" 6  6675.7    6624.4
"1655637D LN Equity" 2015 "1655637D LN Equity2015" 6  7008.2   6841.95
"1655637D LN Equity" 2016 "1655637D LN Equity2016" 6 10459.7   8733.95
"1655637D LN Equity" 2017 "1655637D LN Equity2017" 6       .   5229.85
"1655637D LN Equity" 2018 "1655637D LN Equity2018" 6       .         0
"3572335Q LN Equity" 2003 "3572335Q LN Equity2003" 7 1038.04         .
"3572335Q LN Equity" 2004 "3572335Q LN Equity2004" 7 1281.37 1159.7051
"3572335Q LN Equity" 2005 "3572335Q LN Equity2005" 7 1810.69   1546.03
"3572335Q LN Equity" 2006 "3572335Q LN Equity2006" 7    2574  2192.345
end