VBoxManage see fixed size disk as dynamic

[SAlex]

I have faced with the following problem during the attaching vdi disk to the virtual machine. The disk is fixed size:

VBoxManage showmediuminfo /opt/asm1.vdi
UUID:           8f62813f-0329-426a-8c65-def93934337a
Parent UUID:    base
State:          created
Type:           normal (base)
Location:       /opt/asm1.vdi
Storage format: VDI
Format variant: fixed default
Capacity:       20480 MBytes
Size on disk:   20482 MBytes
Encryption:     disabled

but when I try to attach it:

VBoxManage storageattach node1_rac --storagectl "SATA" --port 1 --device 0 --type hdd --medium asm1.vdi --mtype=shareable
VBoxManage: error: Cannot change type for medium '/opt/asm1.vdi' to 'Shareable' since it is a dynamic medium storage unit
VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MediumWrap, interface IMedium, callee nsISupports
VBoxManage: error: Context: "COMSETTER(Type)(enmMediumType)" at line 708 of file VBoxManageStorageController.cpp
VBoxManage: error: Failed to set the medium type

Could you explain the reason?

[SAlex]

update to 6.0.8 didn't help