This blog explains in more detail why you are probably getting the error and provides two solutions, depending on whether you want to pass just a string or JSON.

Error:

</pre>
{
"errorType": "JsonReaderException",
"errorMessage": "Unexpected character encountered while parsing value: {. Path '', line 1, position 1.",
"stackTrace": [
"at Newtonsoft.Json.JsonTextReader.ReadStringValue(ReadType readType)",
"at Newtonsoft.Json.JsonTextReader.ReadAsString()",
"at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.ReadForType(JsonReader reader, JsonContract contract, Boolean hasConverter)",
"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[T](JsonReader reader)",
"at lambda_method(Closure , Stream , Stream , ContextInfo )"
]
}
</pre>

Solution – If you are passing JSON

I wanted to pass JSON, to at first I built my method like this:

public string HelloWorld(string input, ILambdaContext context) 

I then intended to deserialize the JSON into an object so I could reference each field. I had included my own Deserialize which was totally unneeded, as you will see in a moment:

MajesticData objMajesticData = JsonConvert.DeserializeObject<MajesticData>(jsonMajesticData);

I wanted to reference the fields in the following manner in order to pass parameters values to a SQL stored procedure.

myCommand.Parameters.AddWithValue("@domain", objMajesticData.domain);

My example data (this is what I passing to the Lambda test)

{
   "domain": "irvingseoexpert.com",
   "trustFlow": 18,
   "citationFlow": 25,
   "backlinks": 617,
   "error": null
}

My C# Data Class:

public struct MajesticData
{
   public string domain;
   public DateTime datetime;
   public int trustFlow;
   public int citationFlow;
   public int backlinks;
   public string error;
}

public string SaveMajesticData(MajesticData objMajesticData, ILambdaContext context)

Solution – If you are passing a string

Just pass the string in quotes.

Example, your method matches the HelloWorld example:

public string HelloWorld(string input, ILambdaContext context) 

I hope this explained better than I have seen explained in the AWS forums and on StackOverflow.com. The StackOverflow link mentions that you could use JObject instead of string, then pass {“yourVarname”,”your value”} (where “yourVarname” is the name of the parm and it is of type JObject.