Subnumber has to be numeric (correct entry)

What causes this issue?

You have specified external number assignment in this asset class.
The sub-number must be numeric, even if you are using external
number assignment.

System Response

Sub-number &V1& is not allowed.

How to fix this error?

Enter a numeric sub-number.
Use only the numbers 0-9, without blanks, for the four-place
asset sub-number.

Error message extract from SAP system. Copyright SAP SE.