IdentityServer 3 not validating secret, "invalid_client" response: Implicit flow, revocation endpoint

Question

I'm using oidc-client in my SPA application against IdentityServer3, using implicit flow.

Revoking my reference token on sign out is not working.

I have set revokeAccessTokenOnSignout: true and client_secret to a dummy secret, and I see the request being sent out. However I'm getting a "400 bad request invalid client" response. Here's the request being sent:

And here's the response: {"error":"invalid_client"}

And here's the IdentityServer config:

new Client
            {
                ClientName = "Client Name",
                ClientId = "myclientid",
                ClientSecrets = new List<Secret>
                {
                    new Secret("secret".Sha256())
                },
                AccessTokenType = AccessTokenType.Reference,
                Flow = Flows.Implicit,
                RequireConsent = false,
                RedirectUris = new List<string>
                {
                  ...
                },
                AllowedCorsOrigins = new List<string>
                {
                  ...
                },
                PostLogoutRedirectUris = new List<string>
                {
                     ...
                },
                AllowedScopes = new List<string>
                {
                    ...
                } //.. allowed
            }, //..new client

Any ideas what I might be doing wrong?