AutoMapper Custom Resolver with multi-field key

Question

I've read the AutoMapper docs and the similar threads but I'm either confused or stymied so I turn to my favorite group of experts ;)

I have a lookup table that I load into a Dictionary for use in a CustomResolver. keyValue1 + keyValue2 is my "key".

public class myLookUpEntity
{
    public string keyValue1 { get; set; }
    public string keyValue2 { get; set; }
    public string CodeLookup { get; set; }
}

This loads into a Dictionary<string, myLookupEntity>. Load is just fine via an IOptions pattern to load the table into memory.

I attempt to map from two different source objects to the same destination object using something like this

.ForMember(dest => dest.targetField, opt => opt.MapFrom<MyLookupResolver, string>(src => src.sourceField1.PadRight(14)))
and
.ForMember(dest => dest.targetField, opt => opt.MapFrom<MyLookupResolver, string>(src => "".PadRight(10) + src.sourceField2.PadRight(4)))

The resulting lookup "key" is 14 characters passed to the resolver. Here's the resolver code

public class LookupTable
{
    public IDictionary<string, myLookupEntity> LookupEntries { get; set; }
}

public class MyLookupResolver : IMemberValueResolver<object, object, string, string>
{
    private readonly LookupTable myLookupTable;

    public MyLookupResolver()
    {
        // This constructor is only used by XUnit Tests
        this.myLookupTable = new LookupTable() { LookupEntries = new Dictionary<string, myLookUpEntity>() { } };
    }

    public MyLookupResolver(IOptions<LookupTable> myTable)
    {
        this.myLookupTable = myTable.Value;
    }

    public string Resolve(object source, object destination, string keyValue, string resultValue, ResolutionContext context)
    {
        resultValue = "";

        if (myLookupTable.LookupEntries.TryGetValue(keyValue, out myLookupEntity value))
            resultValue = value.CodeLookup;

        return resultValue;
    }
}

Only problem is, it doesn't work I cannot figure out how to pass two source values to the resolver and let it form the key. Also I am getting this exception

Error mapping types.

Mapping types: sourceObject1 -> destObject .sourceObject1 -> .destObject

Type Map configuration: sourceObject1 -> destObject .sourceObject1 -> .destObject

Destination Member: targetField

Hope this is enough detail to set me on the right path.