A degeneracy in strong lens model is shown analytically. The observed time delays and quasar image positions might not uniquely determine the concentration and the extent of the lens galaxy halo mass distribution. Simply hardwiring the Hubble constant (H_0) and the cosmology (Ω, Λ) to the standard values of ΛCDM cosmology might not fully lift this degeneracy, which exists rigourously even with very accurate data. Equally good fits to the images could be found in lens mass models with either a

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... stly Keplerian or a flat rotation curve. This degeneracy in mass models makes the task of getting reliable H_0 and Λ from strong lenses even more daunting.