[R] Dimension Err using marks in ppp.object

[Michael Fuller]

I want to generate a ppp object (spastat R package) using the command  
'ppp' and 3 vectors containing coordinates and numeric mark data, but  
I get an error I don't understand. The command is:

index10.ppp  <- ppp(x, y, c(20,240), c(140,780), marks=z)

where:
x is a numeric vector of integers that represents the x-coordinate in  
the plane
y is a numeric vector of integers that represents the y-coordinate in  
the plane
z is a numeric vector of doubles (float) that represents the values  
of an index measured at each x-y point
The values given for owin are the corners of the experimental plot

Here is the error message:
"Attempted to create point pattern with 1 columns of mark data;  
multidimensional marks are not yet implemented"

How can 1 column of data be multidimensional? Does this error refer  
to my use of integer coordinates and double marks? Is there a work  
around?

I'm using R for Mac OS X. The version of R is: 1.15, framework R  
2.3.0. The version of spastat is: 1.9-1.

Any advice is appreciated.
MF