"Assertion failed" message on cvs checkout by tag

[George]

I am attempting to do a checkout using the -r option.  This has worked
with this same command many times in the past.
cvs co -r <tag> <module>
Assertion failed: strncmp (repository, CVSroot_directory, strlen
(CVSroot_directory)) == 0, file lock.c, line 176
cvs [server aborted]: received abort signal
cvs [server aborted]: received abort signal
Assertion failed: strncmp (repository, CVSroot_directory, strlen
(CVSroot_directory)) == 0, file lock.c, line 176

What could cause this problem?

Thanks
George