BigInt problem

[tsukikage]

Please see source in attachment.
The output is

  M2 M3 M5 M7 M13 M17 M19 M31 M61 M89 M107 M127 M521 M607 M1279 M2203 M2281 M3217 M4253 M4423
*** M9689***
  M9941 M11213 M19937
*** M21701***
  M23209

It missed 2 Mersenne Primes 9689 & 21701.
Is it my program bug or bigint broken?
It seems subtle.

Thank you!
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