MrBayes - sumt - summarize tree samples in older analyses

Question

[robwhite]

The MrBayes command manual states:

"You can also use the "Sumt" command to summarize tree samples in
older analyses. If you want to do that, remember to first read in a matrix
so that MrBayes knows what taxon names to expect in the trees"

I have copied the run.t files and the .nex file into the same folder as mrbayes_x86.exe but I cannot load the matrix. The message I get when I run the matrix command with my .nex file is:

MrBayes > matrix virgo.nex

Number of taxa and characters needs to be defined before matrix is read

Error when setting parameter "MatrixInfo" (2)

I have examined the .nex file and all of the needed blocks are there. Am I making an obvious error? Any ideas appreciated. Thanks, Rob