When nothing is passed to measure in lexical diversity, TTR is passed as default and an object is created. However, when no arguments are passed to readability, it throws an error - this should behave similarly to lexdiv (ARI should be assigned when empty).
> textstat_lexdiv(dfm(data_char_ukimmig2010))
document TTR
1 BNP 0.3549083
2 Coalition 0.5757576
3 Conservative 0.5176991
4 Greens 0.4950658
5 Labour 0.4474960
6 LibDem 0.5506912
7 PC 0.7128713
8 SNP 0.6532258
9 UKIP 0.4872611
> textstat_readability((data_char_ukimmig2010))
Error in textstat_readability(corpus(x), measure, remove_hyphens, min_sentence_length, :
argument "measure" is missing, with no default 
When nothing is passed to measure in lexical diversity, TTR is passed as default and an object is created. However, when no arguments are passed to readability, it throws an error - this should behave similarly to lexdiv (ARI should be assigned when empty).