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Entangled photon pairs have promised to deliver a substantial quantum advantage for two-photon absorption spectroscopy. However, recent work has challenged the previously reported magnitude of quantum enhancement in two-photon absorption. We present a measurement of molecular absorption driven by isolated photon pairs using sum-frequency generation in a nonlinear optical crystal as a calibration device. We thereby establish an upper bound on the enhancement for entangled two-photon absorption in Rhodamine 6G, which lies well below previously reported values.