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

Filter out NULL entries: filter(entries, e -> e IS NOT NULL) before calling the function.

prestodb/presto · error · PrestoException

INVALID_FUNCTION_ARGUMENT

INVALID_FUNCTION_ARGUMENT

Error message

map entry cannot be null

What it means

multimap_from_entries builds a multimap from an array of ROW(key, value) entries. If any entry in the input array is NULL, the function cannot extract a key/value pair and throws INVALID_FUNCTION_ARGUMENT, since a null map entry has no defined key.

Source

Thrown at presto-main-base/src/main/java/com/facebook/presto/operator/scalar/MultimapFromEntriesFunction.java:65

    @TypeParameter("V")
    @SqlType("map(K,array(V))")
    @SqlNullable
    public Block multimapFromEntries(
            @TypeParameter("map(K,array(V))") MapType mapType,
            @SqlType("array(row(K,V))") Block block)
    {
        Type keyType = mapType.getKeyType();
        Type valueType = ((ArrayType) mapType.getValueType()).getElementType();
        RowType rowType = RowType.anonymous(ImmutableList.of(keyType, valueType));
        int entryCount = block.getPositionCount();
        IntList[] entryIndicesList = new IntList[entryCount];
        for (int i = 0; i < entryIndicesList.length; i++) {
            entryIndicesList[i] = new IntArrayList();
        }
        TypedSet keySet = new TypedSet(keyType, entryCount, NAME);
        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");
            }

            entryIndicesList[keySet.addAndGetPosition(rowBlock, 0)].add(i);
        }

        BlockBuilder multimapBlockBuilder = mapType.createBlockBuilder(null, keySet.size());
        BlockBuilder singleMapWriter = multimapBlockBuilder.beginBlockEntry();
        for (int i = 0; i < keySet.size(); i++) {
            keyType.appendTo(rowType.getObject(block, entryIndicesList[i].getInt(0)), 0, singleMapWriter);
            BlockBuilder singleArrayWriter = singleMapWriter.beginBlockEntry();
            for (int entryIndex : entryIndicesList[i]) {
                valueType.appendTo(rowType.getObject(block, entryIndex), 1, singleArrayWriter);
            }

View on GitHub (pinned to 55bb57d202)

Solutions

Filter out NULL entries: filter(entries, e -> e IS NOT NULL) before calling the function.
Use try_multimap_from_entries to get NULL instead of an error.
Fix upstream queries (COALESCE or JOIN conditions) so entries are never NULL.
Use reduce/if to skip nulls when constructing the array.

Example fix

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

Defensive patterns

Strategy: validation

Validate before calling

SELECT multimap_from_entries(filter(entries, e -> e IS NOT NULL));

Try / catch

SELECT try(multimap_from_entries(entries)) AS mm;

When it happens

Trigger: Calling multimap_from_entries(ARRAY[ROW('a', 1), NULL]) — any NULL element inside the entries array triggers it.

Common situations: Arrays built from LEFT JOINs or array_agg over nullable columns contain NULL elements; JSON-derived arrays include nulls.

Related errors

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/32232ae7b0b1f36c. Report an issue: GitHub.