What causes "map entry cannot be null" in prestodb/presto?

Filter nulls first: map_from_entries(filter(entries, e -> e IS NOT NULL)).

prestodb/presto · error · PrestoException

INVALID_FUNCTION_ARGUMENT

INVALID_FUNCTION_ARGUMENT

Error message

map entry cannot be null

What it means

Thrown by map_from_entries when an element of the input array(row(K,V)) is null. Each entry of the array must be a non-null row so its key and value can be read; a null entry (as opposed to a null value inside an entry) is rejected.

Source

Thrown at presto-main-base/src/main/java/com/facebook/presto/operator/scalar/MapFromEntriesFunction.java:62

    @SqlType("map(K,V)")
    @SqlNullable
    public Block mapFromEntries(
            @TypeParameter("map(K,V)") MapType mapType,
            SqlFunctionProperties properties,
            @SqlType("array(row(K,V))") Block block)
    {
        Type keyType = mapType.getKeyType();
        Type valueType = mapType.getValueType();
        RowType rowType = RowType.anonymous(ImmutableList.of(keyType, valueType));
        int entryCount = block.getPositionCount();

        BlockBuilder mapBlockBuilder = mapType.createBlockBuilder(null, block.getPositionCount());
        BlockBuilder resultBuilder = mapBlockBuilder.beginBlockEntry();
        TypedSet uniqueKeys = new TypedSet(keyType, entryCount, "map_from_entries");

        for (int i = 0; i < entryCount; i++) {
            if (block.isNull(i)) {
                throw new PrestoException(INVALID_FUNCTION_ARGUMENT, "map entry cannot be null");
            }
            Block rowBlock = rowType.getObject(block, i);

            if (rowBlock.isNull(0)) {
                throw new PrestoException(INVALID_FUNCTION_ARGUMENT, "map key cannot be null");
            }

            if (!uniqueKeys.add(rowBlock, 0)) {
                throw new PrestoException(INVALID_FUNCTION_ARGUMENT, format("Duplicate keys (%s) are not allowed", keyType.getObjectValue(properties, rowBlock, 0)));
            }

            keyType.appendTo(rowBlock, 0, resultBuilder);
            valueType.appendTo(rowBlock, 1, resultBuilder);
        }

        mapBlockBuilder.closeEntry();
        return mapType.getObject(mapBlockBuilder, mapBlockBuilder.getPositionCount() - 1);
    }

View on GitHub (pinned to 55bb57d202)

Solutions

Filter nulls first: map_from_entries(filter(entries, e -> e IS NOT NULL)).
Fix upstream query/logic so null rows are not produced.
Use COALESCE/if to replace null entries with default entries where semantically valid.
Keep the call but wrap the pipeline with a fail-safe default map if nulls are expected.

Example fix

// before
SELECT map_from_entries(entries) FROM t;
// after
SELECT map_from_entries(filter(entries, e -> e IS NOT NULL)) FROM t;

Defensive patterns

Strategy: validation

Validate before calling

SELECT count(*) FROM t WHERE any_match(entries, e -> e IS NULL); -- must be 0

Prevention

Filter null entries before map_from_entries
Avoid outer joins that emit null rows into entry arrays
Add assertions in ETL that entries are non-null

When it happens

Trigger: Passing an array(row(K,V)) that contains a NULL element (the whole row is null) to map_from_entries.

Common situations: Array built from outer joins, unnest of sparse data, or CASE expressions that produce null rows.

Related errors

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/861493e2f5be2c83. Report an issue: GitHub.