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The (mostly) insulating behaviour of PrBa2Cu3O7-d is still unexplained and even more interesting since the occasional appearance of superconductivity in this material. Since YBa2Cu3O7-d is nominally iso-structural and always superconducting, we have measured the electron momentum density in these materials. We find that they differ in a striking way, the wavefunction coherence length in PrBa2Cu3O7-d being strongly suppressed. We conclude that Pr on Ba-site substitution disorder is responsible for the metal-insulator transition. Preliminary efforts at growth with a method to prevent disorder yield 90K superconducting PrBa2Cu3O7-d crystallites., Comment: 4 pages, 3 figures, revised version submitted to PRL