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Quasars show considerable promise as standard candles in a high-redshiftwindow beyond Type Ia supernovae. Recently, Risaliti, Lusso \& collaborators\cite{Risaliti:2018reu, Lusso:2019akb, Lusso:2020pdb} have succeeded inproducing a high redshift Hubble diagram ($ z \lesssim 7$) that supports "atrend whereby the Hubble diagram of quasars is well reproduced by the standardflat $\Lambda$CDM model up to $z \sim 1.5-2$, but strong deviations emerge athigher redshifts". This conclusion hinges upon a log polynomial expansion forthe luminosity distance. In this note, we demonstrate that this expansion (or"improvements" thereof) typically can only be trusted up to $z \sim 1.5-2$. Asa result, a breakdown in the validity of the expansion may be misinterpreted asa (phantom) deviation from flat $\Lambda$CDM. We further illustrate the problemthrough mock data examples.

On Problems with Cosmography in Cosmic Dark Ages