PINESCRIPT: I cant put diferent values on an indicator for diferent tickers

I want to put a 10EMA on AAPL and a 50EMA on every other symbol but it keeps saying that my "len" is a series(interger) instead of an interger.

//@version=4
study(title="TEST", shorttitle="TEST", overlay=true)

var int   len = na

if barstate.isfirst
    if syminfo.ticker == "AAPL"
        len := 10
    else
        len := 50

abc=ema(close, len)

plot(abc, color=color.green)