apache/cassandra · error · InvalidRequestException

Non-frozen collections are not allowed inside collections:

Error message

Non-frozen collections are not allowed inside collections: 

What it means

A non-frozen collection (list/set/map) cannot be nested directly inside another collection. Multi-level collections must be explicitly frozen so they are stored as a single immutable blob rather than cell-per-element.

Source

Thrown at src/java/org/apache/cassandra/cql3/CQL3Type.java:897

                AbstractType<?> valueType = values.prepare(keyspace, udts).getType();
                switch (kind)
                {
                    case LIST:
                        return new Collection(ListType.getInstance(valueType, !frozen));
                    case SET:
                        return new Collection(SetType.getInstance(valueType, !frozen));
                    case MAP:
                        assert keys != null : "Got null keys type for a collection";
                        return new Collection(MapType.getInstance(keys.prepare(keyspace, udts).getType(), valueType, !frozen));
                }
                throw new AssertionError();
            }

            private void throwNestedNonFrozenError(Raw innerType)
            {
                if (innerType instanceof RawCollection)
                    throw new InvalidRequestException("Non-frozen collections are not allowed inside collections: " + this);
                else if (innerType.isUDT())
                    throw new InvalidRequestException("Non-frozen UDTs are not allowed inside collections: " + this);
            }

            public boolean referencesUserType(String name)
            {
                return (keys != null && keys.referencesUserType(name)) || values.referencesUserType(name);
            }

            @Override
            public String toString()
            {
                String start = frozen? "frozen<" : "";
                String end = frozen ? ">" : "";
                switch (kind)
                {
                    case LIST: return start + "list<" + values + '>' + end;
                    case SET:  return start + "set<" + values + '>' + end;

View on GitHub (pinned to 88fd0f6a0e)

Solutions

1. Wrap inner collections in frozen<>: `set<frozen<list<int>>>`
2. Restructure the data model into a separate table with clustering keys instead of nested collections
3. Freeze the outer collection entirely if whole-value updates are acceptable

Example fix

// before
CREATE TABLE t (id uuid PRIMARY KEY, nested set<list<int>>);
// after
CREATE TABLE t (id uuid PRIMARY KEY, nested set<frozen<list<int>>>);

boolean hasNestedNonFrozenCollection(String typeDecl) {
    String s = typeDecl.toLowerCase().replaceAll("\\s+", "");
    java.util.regex.Matcher m = java.util.regex.Pattern.compile("<((?!frozen<)[a-z]+<)").matcher(s);
    return m.find();
}

try { session.execute(ddl); } catch (InvalidQueryException e) { if (e.getMessage().startsWith("Non-frozen collections are not allowed inside collections")) { /* wrap inner collection in frozen<> */ } else throw e; }

When it happens

Trigger: Declaring e.g. `set<list<int>>`, `map<text, set<int>>`, or `list<map<text,text>>` without wrapping the inner collection in frozen<> in CREATE TABLE / ALTER TABLE.

Common situations: Very common during schema design when developers try deep nested types (JSON-like structures) in Cassandra; since 2.1.3+ nested collections exist but require frozen inner types.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• frozen<> is only allowed on collections, tuples, and user-de
• Counters are not allowed inside collections:
• Non-frozen UDTs are not allowed inside collections:
• Invalid operation (%s) for frozen list column %s
• Constraint cannot be defined on the column <name> of type <t

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/9c66f01bebb20ea3. Report an issue: GitHub.