gradle/gradle · error · IllegalArgumentException

Cannot encode a null string.

Error message

Cannot encode a null string.

What it means

This encoder deduplicates repeated strings and encodes null via a special NULL_STRING index — but only through writeNullableString. Its writeString still rejects null, because the non-null path writes a dedup index with no null representation.

Source

Thrown at platforms/core-runtime/serialization/src/main/java/org/gradle/internal/serialize/kryo/StringDeduplicatingKryoBackedEncoder.java:110

    @Override
    public void writeBoolean(boolean value) {
        output.writeBoolean(value);
    }

    @Override
    public void writeNullableString(@Nullable CharSequence value) {
        if (value == null) {
            writeStringIndex(NULL_STRING);
            return;
        }
        writeNonnullString(value);
    }

    @Override
    public void writeString(CharSequence value) {
        if (value == null) {
            throw new IllegalArgumentException("Cannot encode a null string.");
        }
        writeNonnullString(value);
    }

    private void writeNonnullString(CharSequence value) {
        String key = value.toString();
        if (strings == null) {
            strings = new Object2IntOpenHashMap<>(1024);
        } else {
            int index = strings.getOrDefault(key, -1);
            if (index != -1) {
                writeStringIndex(index);
                return;
            }
        }

        /*
          Actual stored string indices start from 2 so `0` and `1` can be used as special codes:

View on GitHub (pinned to 534f27719b)

Solutions

1. Route optional strings through writeNullableString(value), which emits the NULL_STRING index.
2. Or guard and substitute "" when empty is an acceptable value.
3. Remove nulls upstream so the non-null path is always valid.

Example fix

// before
encoder.writeString(pathOrNull); // throws "Cannot encode a null string."

// after
encoder.writeNullableString(pathOrNull);

if (value == null) {
    encoder.writeNullableString(null); // emits NULL_STRING index
} else {
    encoder.writeString(value);
}

When it happens

Trigger: Calling writeString(null) on the string-deduplicating encoder; forwarding an optional CharSequence field to writeString instead of writeNullableString before dedup lookup.

Common situations: Long-lived caches that use string deduplication where optional fields were later introduced; the same class of null-handling mistake as the other encoders, discovered only when the field is actually null at runtime.

Related errors

• Cannot encode a null string.
• Cannot encode a null string.
• Expecting the end of nested stream.
• Could not write cache value to '%s'.
• Could not read cache value from '%s'.

AI-assisted analysis of gradle/gradle@534f27719b (2026-08-22). Data as JSON: /api/errors/7cd6295a2278f0c9. Report an issue: GitHub.