Exception thrown when using ResolveCompletion()

When using ResolveCompletion() the following exception is thrown:

Exception thrown: Newtonsoft.Json.JsonSerializationException: Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'OmniSharp.Extensions.LanguageServer.Protocol.Models.CompletionItem' because the type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly.
To fix this error either change the JSON to a JSON object (e.g. {"name":"value"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List that can be deserialized from a JSON array. JsonArrayAttribute can also be added to the type to force it to deserialize from a JSON array.
Path 'result', line 1, position 34.
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.EnsureArrayContract(JsonReader reader, Type objectType, JsonContract contract)
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateList(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, Object existingValue, String id)
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
at Newtonsoft.Json.JsonSerializer.Deserialize(JsonReader reader, Type objectType)
at Newtonsoft.Json.Linq.JToken.ToObject(Type objectType, JsonSerializer jsonSerializer)
at Newtonsoft.Json.Linq.JToken.ToObject[T](JsonSerializer jsonSerializer)
at OmniSharp.Extensions.JsonRpc.ResponseRouter.ResponseRouterReturnsImpl.Returning[TResponse](CancellationToken cancellationToken)

Start by reproducing the failure with ResolveCompletion() and trace the response through ResponseRouterReturnsImpl.Returning(), where the stack trace reports the deserialization error. Compare the response's result shape with the expected CompletionItem type; done means the reported JSON array no longer causes an exception and completion responses deserialize correctly.