getting 500 from Azure SignalR Service

Question

I"m following along with https://learn.microsoft.com/en-us/azure/azure-signalr/signalr-quickstart-rest-api
I created an azure signalr service
I downloaded the sample solutions from https://github.com/aspnet/AzureSignalR-samples
in the ChatRoom solution, I put the connection string from my azure signalr service into the web.config
I run the solution in debug, the web page opens, I enter a name, & I see this error in the chrome console:

Failed to load resource: the server responded with a status of 500 (Internal Server Error)

& here is the url that raises the error:

http://localhost:63025/signalr/negotiate?clientProtocol=2.1&connectionData=%5B%7B%22name%22%3A%22chatsamplehub%22%7D%5D&_=1597769545805