What's missing from the following script to successfully update the value of a global project variable avg$\_$age with the mean of the age column in the heart$\_$disease$\_$imputed$\_$values dataset?

hd$\_$dss$\_$dataset $=$ dataiku.Dataset$("$heart$\_$disease$\_$imputed$\_$values"$)$

hd$\_$data$\_$frame $=$ hd$\_$dss$\_$dataset.get$\_--------()$

vars$\_$dict $=$ project.get$\_$variables$()$

vars$\_$dict$[\text{'}--------\text{'}][\text{'}$avg$\_$age'$]=$ hd$\_$data$\_$frame.age.mean$()$

project.set$\_$variables$($vars$\_$dict$)$

dataset $/$ local

dataframe $/$ local

dataframe $/$ standard

dataset $/$ standard