Serialization Exception in .NET WEB API.

I have an ASP.NET WEB API project in Framework 7 using Entity Framework.
(I would have preferred .NET Core but my manager chose this.)

I'm able to make a successful call if I make my Get verb like this:

public string Get(int id)
{
    User sr = new DataAccess.User();
    using (var ent = new DevDataEntities())
    {
        sr = ent.Users.FirstOrDefault(x => x.UserId == id);
        return sr.DisplayName;            
    }
}

But I get an error when I try any of these methods I get an error:

1.
public User Get(int id)
{
    User sr = new ACH_DataAccess.User();
    using (var entities = new ACH_DevDataEntities())
    {
        sr = entities.Users.FirstOrDefault(x => x.UserId == id);
        return sr;
    }
}

2.
public User Get(int id)
{
    User sr = new ACH_DataAccess.User();
    using (var entities = new ACH_DevDataEntities())
    {
        sr = entities.Users.FirstOrDefault(x => x.UserId == id);              
    }
    return sr;
}

3.public User Get(int id)
{    
    using (var entities = new ACH_DevDataEntities())
    {
        var sr = entities.Users.FirstOrDefault(x => x.UserId == id);
        return sr;
    }
}


Here is the page I get:
<Error>
<Message>An error has occurred.</Message>
<ExceptionMessage>
The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'.
</ExceptionMessage>
<ExceptionType>System.InvalidOperationException</ExceptionType>
<StackTrace/>
<InnerException>
<Message>An error has occurred.</Message>
<ExceptionMessage>
Type 'System.Data.Entity.DynamicProxies.User_9A7F08A3597175E07FDCEE95052760F3C97BAD33B600B22C35ACEF344C52B7D2' with data contract name 'User_9A7F08A3597175E07FDCEE95052760F3C97BAD33B600B22C35ACEF344C52B7D2:http://schemas.datacontract.org/2004/07/System.Data.Entity.DynamicProxies' is not expected. Consider using a DataContractResolver if you are using DataContractSerializer or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to the serializer.
</ExceptionMessage>
<ExceptionType>
System.Runtime.Serialization.SerializationException
</ExceptionType>
<StackTrace>
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiTypeAtTopLevel(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle originalDeclaredTypeHandle, Type graphType) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.DataContractSerializer.WriteObject(XmlWriter writer, Object graph) at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content) at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken) --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__22.MoveNext()
</StackTrace>
</InnerException>
</Error>

When I call it through Postman, I get a "Satus: 400 Bad Request. The request cannot be fullfilled due to bad syntax."

When I step through the debugger, I see that data is returned; but getting it to the browser or to Postman fails.

One of the User properties in the database is varbinary(max) but that comes back as null.
Oddly, when I remove it from the EF model, it continues to appear in the debugger.

I got this to work:

public Dictionary<string, string> Get(int id)
{
    User sr = new ACH_DataAccess.User();
    StringBuilder sb = new StringBuilder();
    using (var entities = new ACH_DevDataEntities())
    {
        sr = entities.Users.FirstOrDefault(x => x.UserId == id);
    }
    sr.AvatarImg = new byte[] { 0, 1, 1, 1, 1, 1, 1, 1, 1, 0 };

    Dictionary<string, string> dctUser = new Dictionary<string, string>();
    dctUser.Add("Display Name", sr.DisplayName);
    dctUser.Add("Email", sr.Email);
    return dctUser;
}

But that seems like a lot of work if the User class has many properties.

The DataContractResolver example here seemed unrealisticly complex for what seems like what should be an easy solution:
https://docs.microsoft.com/en-us/dotnet/framework/wcf/samples/datacontractresolver