varblock issues

I am trying to use varblock to get the variance of my ppp.  I am using it
with Kcross because I am comparing two different species.  My code looks
like the following:

v=varblock(pdat1, fun=Kcross, i="Fragaria virginiana",j="Achillea
millefolium")

I keep getting the following error: 
Error in FUN(X[[1L]], ...) : No points have mark i = Fragaria virginiana

There are many points marked with these specific names, and looking at the
manual I don't need to specify the blocks, any tips on troubleshooting this
issue?

-Alyssa Pontes

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