La multiplicidad no es válida en Role 'X' de la relación 'Y'. Dado que todas las propiedades de Dependent Role aceptan valores NULL, la multiplicidad de Principal Role debe ser '0..1'.
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English version
Multiplicity is not valid in Role 'X' in relationship 'Y'. Because all the properties in the Dependent Role are nullable, multiplicity of the Principal Role must be '0..1'.