Find the block device id of the SD card you are creating an image from:
pi@pi ~ $ sudo blkid /dev/mmcblk0: PTUUID="a6202af7" PTTYPE="dos" /dev/mmcblk0p1: SEC_TYPE="msdos" LABEL="boot" UUID="15CD-3B79" TYPE="vfat" PARTUUID="a6202af7-01" /dev/mmcblk0p2: UUID="13d368bf-6dbf-4751-8ba1-88bed06bef77" TYPE="ext4" PARTUUID="a6202af7-02"