Seems isbig_I needs a smaller idea of "big".

Suspect there's a little more to it than this; somewhere, an int must
be getting turned into a num before being stringified, or similar.
This should bring it out of the range where errors can occur as a
result, however.

colomon++ for reporting, bug hunting etc. for this.

src/vm/jvm/runtime/org/perl6/nqp/runtime/Ops.java

@@ -4156,8 +4156,9 @@ public static String base_I(SixModelObject value, long radix, ThreadContext tc)
 
     public static long isbig_I(SixModelObject value, ThreadContext tc) {
         /* Check if it needs more bits than a long can offer; note that
-         * bitLength excludes sign considerations, thus 63 rather than 64. */
-        return getBI(tc, value).bitLength() > 63 ? 1 : 0;
+         * bitLength excludes sign considerations, thus 32 rather than
+         * 32. */
+        return getBI(tc, value).bitLength() > 31 ? 1 : 0;
     }
 
     public static SixModelObject fromnum_I(double num, SixModelObject type, ThreadContext tc) {