[isabelle] How do I set --maxheap greater than 16G on Linux?

I am using Ubuntu 18.04.  I am finding that the ML heap seems to be limited to 16G and I need to use
a bit more.  I haven't really been able to find a comprehensible description of how to do this in
the documentation I have looked at, so I attempted to follow the settings reported in the Jenkins
nightly build logs, which seem to use --maxheap 20G.  I put the following in ~/.isabelle/etc/settings:

ML_OPTIONS="-H 4000 --maxheap 20G"

However, when I start Isabelle/JEdit I get the message shown below.  How can I get around this?
Thanks for any help.

									-- Gene Stark

Value of --maxheap option must not exceeed 16Gbytes
-H <Initial heap size (MB)>
--minheap <Minimum heap size (MB)>
--maxheap <Maximum heap size (MB)>
--gcpercent <Target percentage time in GC (1-99)>
--stackspace <Space to reserve for thread stacks and C++ heap(MB)>
--gcthreads <Number of threads to use for garbage collection>
--debug <Debug options: checkmem, gc, x>
--logfile <Logging file (default is to log to stdout)>
--exportstats <Enable another process to read the statistics>
Debug options:
checkmem <Perform additional debugging checks on memory>
gc <Log summary garbage-collector information>
gcenhanced <Log enhanced garbage-collector information>
gcdetail <Log detailed garbage-collector information>
memmgr <Memory manager information>
threads <Thread related information>
gctasks <Log multi-thread GC information>
heapsize <Log heap resizing data>
x <Log X-windows information>
sharing <Information from PolyML.shareCommonData>
locks <Information about contended locks>
rts <General run-time system calls>
saving <Saving and loading state; exporting>
Unfinished session(s): HOL, Pure
Return code: 1

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