Polish signs in shinydashboard

I've got problem with polish signs in shinydashboard. When I'm using just shiny everything is ok, but in shinydashboard ą and ę are written in different font. I've tried to change encoding of a document, but it didn't help. I don't really know how to deal with it... Could you help me?

My code:

library("shiny")
library("shinydashboard")

ui <- dashboardPage(
    dashboardHeader(),
    dashboardSidebar(),
    dashboardBody(h1("przejścią"))
)

server <- function(input, output) { }

shinyApp(ui, server)

And visualisation of my problem: