Informix Error -789: Internal error, expression not properly defined.

Reasons / Common Causes

-789 is flagged by the official text as an internal error: an expression the server is processing isn't properly defined internally, with no documented application-level trigger.

1. A genuine internal engine condition in expression handling, per the official guidance's framing, with no other documented application-level cause.
2. An unusually complex or malformed-looking expression (even if it parsed successfully) that exercises an uncommon internal code path, the same general category as -788.

Solutions / Resolution

1. If you cannot find a direct cause, note all circumstances and contact IBM Informix Technical Support, per the official guidance — there is no documented general application-level fix.
2. Try simplifying the expression that triggered the failure, as a practical first step.
3. Capture the exact statement/expression for escalation, since the internal condition is unlikely to be diagnosable from the error text alone.

Examples

Capturing circumstances for escalation

-- Record the exact expression that produced -789, and try simplifying it
-- to isolate the specific construct involved.

Diagnostic Checks

1. Try simplifying the expression to isolate the specific construct triggering the failure.
2. Capture the exact statement for escalation if simplification doesn't help.

Related Errors / Related Topics

• -788 — "Unknown operator/type." A related internal expression-handling error, in the same general area.

An internal expression-definition error — try simplifying the expression first, then escalate with the exact statement captured.