Why do I get "Invalid Taproot control block size" when spending a P2TR script path?

I'm building a Taproot script-path spend in Python (using bitcoin-utils or custom serialization helpers) and testmempoolaccept keeps rejecting with Invalid Taproot control block size. (The exact wrapper varies—sometimes non-mandatory-script-verify-flag, etc.)

Checked everything: tapleaf hash , Merkle path , hash count matches the tree. Preimage/script look fine. So I'm pretty sure the content of the control block is ok.

What I'm doing now:

control_block = [
    leaf_version,
    internal_pubkey,
    merkle_hash_1,
    merkle_hash_2
]

control_block_bytes = ser_string(b''.join(control_block))  # compactSize prefix

witness_stack = [
    preimage_or_sig,
    script,
    control_block_bytes
]

Tried splitting the control block into separate stack elements too (leaf_version, internal_key, hash1, hash2...) — that gave bad-witness-nonstandard instead.

What does "Invalid Taproot control block size" actually mean here? Am I serializing the control block wrong?



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