Error when using expt.

I get this error instead of getting a value when running this code.
SISC (1.16.6)
#;> (expt 2 (expt 3 (expt 4 5)))
Error in ashl: exact integer required for operation.
---------------------------
To enable more detailed stack tracing, set the dynamic parameter
max-stack-trace-depth to a non-zero value, e.g. 16.
---------------------------
Some stack trace entries may have been suppressed. To see all entries
set the dynamic parameter suppressed-stack-trace-source-kinds to '().
#;>

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