SAP Message AA196 - Subnumber has to be numeric (correct entry)

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

System Response
Sub-number &V1& is not allowed.

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