Why nrow function doesn t seem to work within summarise function

Question

See the code below:

library(dslabs) \sdata(gapminder)
gapminder percent > percent group by(year) percent > percent summarise(weight = nrow(population)) percent > percent filter(year percent in percent c(2010, 2015) &!is.na(population))

I keep getting the message, "Column weight is of unsupported type NULL," why?

Population is a vector inside the summarise() function, and nrow() accepts a vector as a parameter. I therefore wonder why it doesn't.

Note: I am not searching for workable substitutes for nrow() because I am aware of its alternatives in this situation. My query is philosophical and is focused on the word "Why."