diff --git a/src/crypto-api/index.ts b/src/crypto-api/index.ts
index ea6177f7f8..46fe7528a0 100644
--- a/src/crypto-api/index.ts
+++ b/src/crypto-api/index.ts
@@ -274,7 +274,13 @@ export interface CryptoApi {
     isCrossSigningReady(): Promise<boolean>;
 
     /**
-     * Get the ID of one of the user's cross-signing keys.
+     * Get the ID of one of the user's cross-signing keys, if both private and matching
+     * public parts of that key are available (ie. cached in the local crypto store).
+     *
+     * The public part may not be available if a `/keys/query` request has not yet been
+     * performed, or if the device that created the keys failed to publish them.
+     *
+     * If either part of the keypair is not available, this will return `null`.
      *
      * @param type - The type of key to get the ID of.  One of `CrossSigningKey.Master`, `CrossSigningKey.SelfSigning`,
      *     or `CrossSigningKey.UserSigning`.  Defaults to `CrossSigningKey.Master`.