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'.
.Net Framework