Instantiating AmazonCognitoIdentityProviderClient in Unity (WebGL build) gives the error "Value cannot be null. Parameter name: stream"

Question

I am trying to build a unity project for WebGL. The project includes AWS SDK and the dll's are added into the project as in here https://docs.aws.amazon.com/sdk-for-net/v3/developer-guide/unity-special.html.

The project builds fine without errors and generates the web content. When I run it, at the point where I instantiate AmazonCognitoIdentityProviderClient with

Provider = new AmazonCognitoIdentityProviderClient(new AnonymousAWSCredentials(), cognitoRegion);

it throws an exception:

ArgumentNullException: Value cannot be null.
Parameter name: stream
  at System.IO.StreamReader..ctor (System.IO.Stream stream, System.Text.Encoding encoding, System.Boolean detectEncodingFromByteOrderMarks, System.Int32 bufferSize, System.Boolean leaveOpen) [0x00000] in <00000000000000000000000000000000>:0 

Please let me know if anyone else encountered this and whether you found any solution for this.