No entry in conversion table for external segment name &1

What causes this issue?

The external segment name '&v1&' has arisen in an IDoc with a format
used before Release 4.0. An attempt was made to establish the internal
segment name for this. This failed. No entry found in the conversion
table.

System Response

This IDoc cannot be processed.

How to fix this error?

Please check whether an entry is missing in the conversion table for
segments or whether the segment is not yet defined in the system.

Error message extract from SAP system. Copyright SAP SE.