ExecutAsync of TableOperation exceptions aren't consistent

Question

I was testing around the Azure table store in C# and I came across some odd behavior. If I do this:

try {

    // First, create the client
    var account = CloudStorageAccount.Parse(connectionString);
    var client = account.CreateCloudTableClient(new TableClientConfiguration());

    // Next, create the table operation to insert or replace the item
    var operation = TableOperation.Insert(new TableEntity(), true);

    // Now, execute the operation
    CloudTable table = Client.GetTableReference(TableName);
    await table.CreateIfNotExistsAsync().ConfigureAwait(false);
    await table.ExecuteAsync(operation, token).ConfigureAwait(false);
} catch (StorageException e) {
    // This executes
} catch (ArgumentNullException e) {
    // This does not execute
}

then I'll get a StorageException stating that The values are not specified for all properties in the entity.

However, if I do this:

try {

    // First, create the client
    var account = CloudStorageAccount.Parse(connectionString);
    var client = account.CreateCloudTableClient(new TableClientConfiguration());

    // Next, create the table operation to insert or replace the item
    var operation = TableOperation.Merge(new TableEntity {ETag: "*"}, true);

    // Now, execute the operation
    CloudTable table = Client.GetTableReference(TableName);
    await table.CreateIfNotExistsAsync().ConfigureAwait(false);
    await table.ExecuteAsync(operation, token).ConfigureAwait(false);
} catch (StorageException e) {
    // This does not execute
} catch (ArgumentNullException e) {
    // This executes
}

then I get an ArgumentNullException stating that Value cannot be null. (Parameter 'Merge requires a valid PartitionKey').

I would expect similar data conditions to produce similar errors on add and update so why do I get a StorageException when the partition key is empty on insert but an ArgumentNullException when the partition key is empty on merge?