Unexpected Result from bessel_zero_Jnu with NaN input

[Jackson Vanover]

Using version 2.6 of GSL from ftp://ftp.gnu.org/gnu/gsl/gsl-2.6.tar.gz

OS is Ubuntu 18.04.2 LTS

Hardware is a Dell workstation with a 3.60 GHz Intel i7-4790 and 32 GB
of RAM

Compiler is gcc 7.4.0 with the -w option

Description: when the second argument to gsl_sf_bessel_zero_Jnu is a
NaN, that NaN is dropped and the function returns a zero without
complaining. The same is true of an inf input.

#include <gsl/gsl_sf.h>
#include <stdio.h>
#include <gsl/gsl_math.h>

int main (void){
        double out;
        out = gsl_sf_bessel_zero_Jnu(1, GSL_NAN);

        printf("%f\n", out);
        printf("%.*e\n", out);
        return 0;
}

Best,
Jackson Vanover