diff --git src/core/job.c src/core/job.c
index 85f77e8..67e55e6 100644
--- src/core/job.c
+++ src/core/job.c
@@ -335,6 +335,9 @@ static const JobType job_merging_table[] = {
 
 JobType job_type_lookup_merge(JobType a, JobType b) {
         assert_cc(ELEMENTSOF(job_merging_table) == _JOB_TYPE_MAX_MERGING * (_JOB_TYPE_MAX_MERGING - 1) / 2);
+        if (a < 0 || a >= _JOB_TYPE_MAX_MERGING ||
+            b < 0 || b >= _JOB_TYPE_MAX_MERGING)
+                log_error("a=%d b=%d", a, b);
         assert(a >= 0 && a < _JOB_TYPE_MAX_MERGING);
         assert(b >= 0 && b < _JOB_TYPE_MAX_MERGING);