From type/Rational

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method isNaN(Rational:D: --> Bool:D)

Tests whether the invocant's Num value is a NaN, an acronym for Not available Number. That is both its numerator and denominator are zero.

From type/Complex

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method isNaN(Complex:D: --> Bool:D)

Returns true if the real or imaginary part is NaN (not a number).

say (NaN+5i).isNaN; # OUTPUT: «True␤» 
say (7+5i).isNaN;   # OUTPUT: «False␤»