diff --git a/pczt/src/roles/signer/mod.rs b/pczt/src/roles/signer/mod.rs
index b929e5464..a75b86dcf 100644
--- a/pczt/src/roles/signer/mod.rs
+++ b/pczt/src/roles/signer/mod.rs
@@ -139,6 +139,9 @@ impl Signer {
 
     /// Signs the Sapling spend at the given index with the given spend authorizing key.
     ///
+    /// Requires the spend's `proof_generation_key` field to be set (because the API does
+    /// not take an FVK).
+    ///
     /// It is the caller's responsibility to perform any semantic validity checks on the
     /// PCZT (for example, comfirming that the change amounts are correct) before calling
     /// this method.
@@ -179,6 +182,8 @@ impl Signer {
 
     /// Signs the Orchard spend at the given index with the given spend authorizing key.
     ///
+    /// Requires the spend's `fvk` field to be set (because the API does not take an FVK).
+    ///
     /// It is the caller's responsibility to perform any semantic validity checks on the
     /// PCZT (for example, comfirming that the change amounts are correct) before calling
     /// this method.