What does this Azure Search indexing error mean? "The property 'x' does not exist on type 'search.documentFields'..."

Question

The Exception I am getting is: The property 'documentType' does not exist on type 'search.documentFields'. Make sure to only use property names that are defined by the type.

I have googled this and still cannot figure out what is going on.

Here is the model we are using:

[SerializePropertyNamesAsCamelCase]
public class WebSearchDocument : SearchDocument, IEventSearchDocument, IResourceSearchDocument
{
    [IsFacetable]
    public string DocumentType { get; set; }
    [IsSearchable]
    public string Title { get; set; }
    [IsSearchable]
    public string Description { get; set; }
    [IsFilterable]
    public DateTime? PublishedDate { get; set; }
    public DateTime? LastUpdatedDate { get; set; }
    public string ImageUrl { get; set; }
    public string LinkToResource { get; set; }
    public string EventCode { get; set; }
    [IsSearchable, IsFilterable]
    public string Location { get; set; }
    [IsFilterable]
    public DateTime? EventStartDate { get; set; }
    [IsFilterable]
    public DateTime? EventEndDate { get; set; }
}

And finally, this is the index fields on Azure

Per @Bruce Johnson request, here is more information

Service Name: bacp-search
Index in question bacp-web-dev
We are using Microsoft.Azure.Search 3.0.3 (NuGet)

SearchDocument Base:

[SerializePropertyNamesAsCamelCase]
public abstract class SearchDocument : ISearchDocument
{
    /// <summary>
    /// Gets or sets the ID for the document. For consistency, this should never be updated or retrieved manually.
    /// All IDs should be set through the <see cref="Id"/> property.
    /// </summary>
    [Key]
    [JsonProperty("Id")]
    public string AzureId { get; set; }

    /// <summary>
    /// Gets or sets any unique IDs or compound IDs that might contain characters unsafe for transmission via URL.
    /// For consistency, all IDs should be set through this property.
    /// </summary>
    [JsonIgnore]
    [IsRetrievable(false)]
    public string Id
    {
        get
        {
            return AzureId.FromBase64EncodedString();
        }
        set
        {
            AzureId = value.ToBase64EncodedString();
        }
    }
}

Answer 1

In general, this error means that a property in the indexing payload is not present in the index definition. In this specific instance, it's because the indexing request is being sent to the wrong index, and the target index in fact does not have a documentType field.