White image saved with stbi_write_bmp has black columns. Why?

I created a white image with this code:

#include "stb_image/stb_image.h"
#include "stb_image/stb_image_write.h"

int main(void){
    int width = 1440;
    int height = 1080;
    int j, l;
    float *img_slopes = (float *)malloc(width * height * sizeof(float));
    for (l = 0; l < width; l++) {
        for (j = 0; j < height; j++) {
            *(img_slopes + j * width + l) = 255;
        }
    }
    stbi_write_bmp("answer_blank.bmp", width, height, 1, img_slopes);
}

Those cycles should scroll through each and every pixel. But the output is:

png screen-capture of the answer_blank.bmp

Why is that? Why do I have black columns? What exactly is happening here?

If it's not clear enough from the print-screen, every other column is black.

As user @Gerhadh kindly mentioned, the .bmp don't use float vales. It should be an unsigned char matrix instead.