Why is typescript saying that the action payload is possibly undefined in the extraReducer in a slice? #766

export const login = createAsyncThunk<
    Tokens, // Return type of the payload creator
    Credentials, // First argument to the payload creator
    {
        dispatch: AppDispatch;
        state: RootState;
        extra: undefined;
        rejectValue: LoginError; // The type of the response I get from the server
    }
>('auth/login', async (credentials: Credentials, thunkAPI) => {
    try {
        // I've shortened it for the sake of brevity. But if this is relevant, I've included the code I've used below
        const response = await client.post<LoginResponse>('login', credentials);
        const { accessToken, refreshToken } = response.data;
        return { accessToken, refreshToken };
    } catch (e) {
        const error = e as AxiosError<LoginError>;
        const response = error.response as AxiosResponse<LoginError>;
        return thunkAPI.rejectWithValue(response.data);
    }
});

export const authSlice = createSlice({
    name: 'auth',
    initialState,
    extraReducers: builder => {
        builder.addCase(login.pending, (state, action) => {
            state.status = 'pending';
        });
        builder.addCase(login.fulfilled, (state, action) => {
            state.status = 'loggedIn';
        });
        builder.addCase(login.rejected, (state, action) => {
            state.status = 'failed';
            state.error = action.payload; // Here the typescript compiler says: "action.payload is possibly 'undefined'". The type of state.error is LoginError | null
        });
    }
});

const params = new URLSearchParams();
    params.append('scope', 'openid profile offline_access whee_api email');
    params.append('username', encodeURIComponent(credentials.usernameOrEmail));
    params.append('password', encodeURIComponent(credentials.password));
    params.append('client_id', 'whee_mobile');
    params.append('grant_type', 'password');
    try {
        const response = await identityClient.post<LoginResponse>('login', params);
        const { accessToken, refreshToken } = response.data;
        return { accessToken, refreshToken };
    } catch (e) {
        const error = e as AxiosError<LoginError>;
        const response = error.response as AxiosResponse<LoginError>;
        return thunkAPI.rejectWithValue(response.data);
    }