MoreLikeThis with setBoost(true) divides every term's score by the lowest selected score, so a selected term that scores 0 (a TFIDFSimilarity whose idf is 0, such as the textbook log(N/df) for a term in every document) makes like() throw 'boost must be a positive float, got NaN' (8.0 and later) or build NaN and Infinity boosts (4.10 to 7.x)
Finding LFV-559Area: queries / MoreLikeThis boosted query construction (MoreLikeThis.createQuery boost normalisation)Priority: lowImpact: crashNovelty: new
AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.
What it shows
You enable boosting on MoreLikeThis (setBoost(true)) with a custom TFIDFSimilarity whose idf is 0 for a term that is in every document, as the textbook idf log(N/df) is, and call like(...) on text that contains that term and a rarer one. You expect a query with finite, non-negative boosts: the same call works with setBoost(false), and BoostQuery accepts a boost of 0. Instead createQuery divides each term's score by the first score popped from its ascending queue, which is the lowest selected score (0 here), so the zero-score term's boost is 0/0 = NaN and like() throws IllegalArgumentException 'boost must be a positive float, got NaN' on 8.0 and later, or builds the query with NaN and Infinity boosts on 4.10 to 7.x.
The test shows that MoreLikeThis with setBoost(true) and a TFIDFSimilarity whose idf is 0 for a term in every document divides by that zero lowest score, so like() throws.
Expected vs actual
Expected a query with finite boosts (as with setBoost(false); BoostQuery accepts 0); actual IllegalArgumentException "boost must be a positive float, got NaN" from MoreLikeThis.createQuery, with -ea and -da.
Files
Run it
- Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
- Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
- Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
- How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
- Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.
# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar # JUnit Platform console launcher (Maven Central)
javac -d classes -cp "$CP" $(find src -name '*.java')
# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
-cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
execute --select-class org.apache.lucene.queries.mlt.TestMoreLikeThisBoostZeroScoreGradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).
Formal models and scripts this finding's part of the hunt used: queries-mlt-commonterms-reference
Feedback
Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.
Snapshot of 2026-11-03. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.