Likehood ratio test showing error

Dear colleagues, can someone kindly assist me in identifying problem while you likelihood ratio test in this data.Am getting the error message "df(unrestricted) = df(restricted) = 3".

Code:

* Example generated by -dataex-. To install: ssc install dataex
clear
input int yoa double midyrpop str3 spn_serotype byte n str12 type double(time vaxera postslope) byte(_est_a _est_b)
1999              33620 "13" 0 "non-vaccine"  1 0 0 1 1
2000              34505 "13" 1 "non-vaccine"  2 0 0 1 1
2001              35411 "13" 1 "non-vaccine"  3 0 0 1 1
2002              36340 "13" 0 "non-vaccine"  4 0 0 1 1
2003              39747 "13" 0 "non-vaccine"  5 0 0 1 1
2004              41985 "13" 0 "non-vaccine"  6 0 0 1 1
2005              42738 "13" 0 "non-vaccine"  7 0 0 1 1
2006              43916 "13" 0 "non-vaccine"  8 0 0 1 1
2007              44536 "13" 0 "non-vaccine"  9 0 0 1 1
2008              44820 "13" 0 "non-vaccine" 10 0 0 1 1
2009              46343 "13" 0 "non-vaccine" 11 0 0 1 1
2010              47714 "13" 0 "non-vaccine" 12 0 0 1 1
2012  40730.05464480874 "13" 0 "non-vaccine" 13 1 1 1 1
2013 43214.202739726024 "13" 0 "non-vaccine" 14 1 2 1 1
2014              47807 "13" 0 "non-vaccine" 15 1 3 1 1
2015              47921 "13" 0 "non-vaccine" 16 1 4 1 1
2016  42048.96721311475 "13" 0 "non-vaccine" 17 1 5 1 1
2017  21958.98082191781 "13" 0 "non-vaccine" 18 1 6 1 1
2018              46210 "13" 0 "non-vaccine" 19 1 7 1 1
2019              45972 "13" 2 "non-vaccine" 20 1 8 1 1
end

My code is :
tsset time
glm n vaxera time postslope , link(log) family(nbinomial) vce(hac nwest 2) exp(midyrpop) eform
estimate store a
glm n vaxera time , link(log) family(nbinomial) vce(hac nwest 2) exp(midyrpop) eform
estimate store b
lrtest a b