Fix SSZ underflow

tests/core/pyspec/eth2spec/test/helpers/random.py

@@ -53,7 +53,7 @@ def exit_random_validators(spec, state, rng, fraction=0.5, exit_epoch=None, with
     """
     if from_epoch is None:
         from_epoch = spec.MAX_SEED_LOOKAHEAD + 1
-    epoch_diff = from_epoch - spec.get_current_epoch(state)
+    epoch_diff = int(from_epoch) - int(spec.get_current_epoch(state))
     for _ in range(epoch_diff):
         # NOTE: if `epoch_diff` is negative, then this loop body does not execute.
         next_epoch(spec, state)