DRBD Failure: (127) Device minor not allocated

I use wmware workstation to run two virtual machines with OpenVZ 2.6.32-042stab108.2 installed on top on CentOS 6.6. I have created another primary partition, /dev/sda4, to configure it as drbd resource. I also created a filsystem on it. The second machine is actually created using the virtual disk of the first one, with changed hostname and eth0 ip address. The drbd configuration file is this:

global { usage-count no; } 
common { syncer { rate 100M; } } 
resource r0 { 
        protocol C; 
        startup { 
                wfc-timeout  15; 
                degr-wfc-timeout 60; 
        } 
        net { 
                cram-hmac-alg sha1; 
                shared-secret "password"; 
        } 
        on primary { 
                device /dev/drbd0; 
                disk /dev/sda4; 
                address 192.168.18.10:7788; 
                meta-disk internal; 
        } 
        on secondary { 
                device /dev/drbd0; 
                disk /dev/sda4; 
                address 192.168.18.20:7788; 
                meta-disk internal; 
        } 
}

After creating the resource with drbdadm create-md r0, when I enter service drbd start, I get:

Failure: (127) Device minor not allocated.

The output of drbdadm dump all might be helpful:

[root@primary ~]# drbdadm dump all
# /etc/drbd.conf
# resource r0 on primary: not ignored, not stacked
resource r0 {
    protocol               C;
    on primary {
        device           /dev/drbd0 minor 0;
        disk             /dev/sda4;
        address          ipv4 192.168.18.10:7788;
        meta-disk        internal;
    }
    on secondary {
        device           /dev/drbd0 minor 0;
        disk             /dev/sda4;
        address          ipv4 192.168.18.20:7788;
        meta-disk        internal;
    }
    net {
        cram-hmac-alg    sha1;
        shared-secret    danuts;
    }
    startup {
        wfc-timeout       15;
        degr-wfc-timeout  60;
    }
}

What is causing this error and how can it be mitigated? Thanks!