Reference type in EF query causing strange OR (1 = 0) in WHERE clause

I have the following setup:

private IQueryable<ManagementCreditorEntity> CreateQuery(ExternalId creditorId)
{
    //_creditorManagedPropertyDbContext is of type ICreditorManagedPropertyDbContext
    return _creditorManagedPropertyDbContext.ManagementCreditorEntities
        .Where(entity => entity.Creditor.ExternalId == creditorId);
}

public interface ICreditorManagedPropertyDbContext
{           
    IDbSet<ManagementCreditorEntity> ManagementCreditorEntities { get; }
}

public class CreditorEntity
{
    public short CompanyId { get; set; }
    public int CreditorId { get; set; }
    public Guid ExternalId { get; set; }
}

public class ManagementCreditorEntity
{
    public short CompanyId { get; set; }
    public int CreditorId { get; set; }
    public CreditorEntity Creditor { get; set; }
}

public sealed class ExternalId
{
    private readonly Guid _value;

    public ExternalId(Guid id)
    {
        _value = id;
    }

    public static implicit operator Guid(ExternalId id)
        => id?._value ?? Guid.Empty;
}

I'm calling the above using:

var query = CreateQuery(new ExternalId(Guid.NewGuid()));

This results in the the following SQL query:

-- With ExternalId as the param type for CreateQuery(), note the "OR (1 = 0)" at the end
SELECT 
    [Extent1].[CompanyID] AS [CompanyID], 
    [Extent1].[CreditorID] AS [CreditorID],    
    FROM  [PropertyManagement].[ManagementCreditor] AS [Extent1]
    INNER JOIN [PropertyManagement].[Creditor] AS [Extent2] ON ([Extent1].[CreditorID] = [Extent2].[CreditorID]) AND ([Extent1].[CompanyID] = [Extent2].[CompanyID])
    WHERE ([Extent2].[ExternalID] = @p__linq__0) OR (1 = 0)

I noticed that if I change the param type of CreateQuery() from ExternalId to Guid, the query is the same except the redundant OR (1 = 0) is removed.

-- With Guid as the param type for CreateQuery()    
SELECT 
    [Extent1].[CompanyID] AS [CompanyID], 
    [Extent1].[CreditorID] AS [CreditorID], 
    FROM  [PropertyManagement].[ManagementCreditor] AS [Extent1]
    INNER JOIN [PropertyManagement].[Creditor] AS [Extent2] ON ([Extent1].[CreditorID] = [Extent2].[CreditorID]) AND ([Extent1].[CompanyID] = [Extent2].[CompanyID])
    WHERE [Extent2].[ExternalID] = @p__linq__0

Why does OR (1 = 0) get added if I wrap a Guid in a reference type (ExternalId)?

I'm using .NET 4.6.2 and Entity Framework 6.2.0.