Placing an ASSERT in a SQL Server query

Question

As part of a MERGE query that I want to run I'd like to assert at runtime that a certain condition holds. When a MERGE-match is found I'd like to update a certain column and have the following logic executed:

1. If the target column is NULL, write the source value
2. If the target is NOT NULL, assert that target and source are identical

I expect that the two values are always identical in case 2 but I might have made a mistake (have a bug). When that happens I'd like to crash the statement and let my app report the error. This is a very rare error condition, not something that can happen as part of usual processing.

So I was thinking that I could abuse a divide-by-zero exception to trigger a crash:

MERGE
...
WHEN MATCHED BY TARGET THEN UPDATE SET
  TargetCol = CASE
    WHEN TargetCol IS NULL THEN SourceCol
    WHEN TargetCol = SourceCol THEN SourceCol
    ELSE 0/0 END --crash!

Will this work reliably? Is there a reason this should not be done?