hello, i dont know why stata keeps me showing invalid syntax in this loop:

foreach x of p524a1_14 p524a1_15 p524a1_16 p524a1_17 p524a1_18{
summ `x'
gen ING_`x' = (`x'-r(mean))/r(sd)
}

i tried with an space just before the "{" and every way but i cant do it
please send your suggestion
thank you so much