diff --git a/bitcoin/src/blockdata/transaction.rs b/bitcoin/src/blockdata/transaction.rs
index fc88001b1..6182308f5 100644
--- a/bitcoin/src/blockdata/transaction.rs
+++ b/bitcoin/src/blockdata/transaction.rs
@@ -784,6 +784,8 @@ pub fn effective_value(
     let weight = input_weight_prediction.total_weight();
     let fee = fee_rate.to_fee(weight);
 
+    // Cannot overflow because after conversion to signed Amount::MIN - Amount::MAX
+    // still fits in SignedAmount::MAX (0 - MAX = -MAX).
     (value.to_signed() - fee.to_signed()).expect("cannot overflow")
 }