Google Firebase SAML Auth: Getting 400 Error INVALID_IDP_RESPONSE: has no assertions

I am trying to connect my Firebase application with Stanford's SUNet as an identity provider. I'm getting a 400 response after successful login via popup. Relevant code pasted below. Has anyone seen this before?

  const samlLogin = (samlProvider) => () => {
    const auth = firebase.auth();
    const provider = new firebase.auth.SAMLAuthProvider(samlProvider);
    auth.signInWithPopup(provider).then((userCredential) => {
      console.log(userCredential);
      onSuccess(userCredential.user);
    })
    .catch((err) => {
      console.log(err);
      setLoading(false);
      setErrorText('Something went wrong. Try again or contact support.')
    });
  }

domain: "global" message: "INVALID_IDP_RESPONSE : Invalid Response: has no assertions." reason: "invalid"