ARB_copy_buffer weird behavior

#include <stdlib.h>
#include <string.h>

#include <GL/glew.h>
#include <SDL2/SDL.h>

static SDL_Window *win;
static SDL_GLContext *ctx;

void setupGL()
{
    SDL_Init(SDL_INIT_VIDEO);
    win = SDL_CreateWindow("CopyBufferBug", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 64, 64, SDL_WINDOW_OPENGL);
    ctx = SDL_GL_CreateContext(win);
    glewInit();
}

static void teardownGL()
{
    SDL_GL_DeleteContext(ctx);
    SDL_DestroyWindow(win);

    SDL_Quit();
}

int main(int argc, char *argv[])
{
    setupGL();

    /* These don't matter I think */
    #define BLOCK_SIZE 128
    #define BUFFER1_SIZE BLOCK_SIZE
    #define BUFFER2_SIZE BLOCK_SIZE
    #define BUFFER1_TARGET GL_COPY_READ_BUFFER
    #define BUFFER2_TARGET GL_COPY_WRITE_BUFFER
    #define BUFFER1_USAGE GL_DYNAMIC_DRAW
    #define BUFFER2_USAGE GL_DYNAMIC_DRAW

    GLuint buffers[2];
    glGenBuffers(2, buffers);

    /* We allocate both buffers with undefined memory */
    glBindBuffer(BUFFER1_TARGET, buffers[0]);
    glBufferData(BUFFER1_TARGET, BUFFER1_SIZE, 0, BUFFER1_USAGE);

    glBindBuffer(BUFFER2_TARGET, buffers[1]);
    glBufferData(BUFFER2_TARGET, BUFFER2_SIZE, 0, BUFFER2_USAGE);

    /* Then copy (undefined) bytes from the first into the second buffer */
    /* Note: If I comment this line out, everything works */
    glCopyBufferSubData(BUFFER1_TARGET, BUFFER2_TARGET, 0, 0, BUFFER1_SIZE);

    /* Generate random string */
    FILE *rand = fopen("/dev/urandom", "r");
    char data[BLOCK_SIZE];
    fread(data, 1, sizeof(data), rand);
    fclose(rand);

    /* We fill the second buffer with defined data */
    /* Note: If I execute this call twice (just copy paste the line), everything works */
    glBufferSubData(BUFFER2_TARGET, 0, sizeof(data), data);

    /* Then download it again to compare its contents against client buffer */
    char data2[BLOCK_SIZE];
    glGetBufferSubData(BUFFER2_TARGET, 0, sizeof(data2), data2);

    if (memcmp(data, data2, sizeof(data)))
        printf("Data does NOT match up!\n");
    else
        printf("Data matches.\n");

    glDeleteBuffers(2, buffers);

    teardownGL();

    return 0;
}