When are scala's for-comprehensions lazy?

In Python, I can do something like this:

lazy = ((i,j) for i in range(0,10000) for j in range(0,10000))
sum((1 for i in lazy))

It will take a while, but the memory use is constant.

The same construct in scala:

(for(i<-0 to 10000; j<-i+1 to 10000) yield (i,j)).count((a:(Int,Int)) => true)

After a while, I get a java.lang.OutOfMemoryError, even though it should be evaluated lazily.