No feedback if mount command fails when executed with popen() function

Question

When executing the linux command "mount", with invalid parameters and execute it with the popen() function in my C++ code. The output is empty. I'm expecting an error like this: "mount: mount point test does not exist"

Here is the code that should generate some errors as 'test' doesn't exist:

FILE *file;
char data[512];
if((file = popen("mount test test", "r")))
{
    while(fgets(data, sizeof(data), file)!=NULL)
    {
         printf("%s", data);
    }
    pclose(file);
}

data array is empty. But when I replace the command "mount test test" with "date" the data array gets filled with the following: "Wed Jan 4 08:20:23 EST 2017".

Thanks if any of you have ideas.