cuda unknown error

gistfile1.md

masaki@desktop:~/ $ cat main.cu

#include <stdio.h>

int main() {
    int count = 0;
    cudaError_t error;

    error = cudaGetDeviceCount(&count);
    printf("count = %d\nerror = %s\n", count, cudaGetErrorString(error));
    return 0;
}

masaki@desktop:~/ $ /usr/local/cuda/bin/nvcc main.cu
masaki@desktop:~/ $ ./a.out
count = 0
error = unknown error