diff --git a/src/script/standard.cpp b/src/script/standard.cpp
index 063e149d36c..5d808914854 100644
--- a/src/script/standard.cpp
+++ b/src/script/standard.cpp
@@ -485,6 +485,7 @@ WitnessV1Taproot TaprootBuilder::GetOutput() { return WitnessV1Taproot{m_output_
 TaprootSpendData TaprootBuilder::GetSpendData() const
 {
     assert(IsComplete());
+    assert(m_output_key.IsFullyValid());
     TaprootSpendData spd;
     spd.merkle_root = m_branch.size() == 0 ? uint256() : m_branch[0]->hash;
     spd.internal_key = m_internal_key;