diff --git a/keyserver/storage/postgres/cross_signing_keys_table.go b/keyserver/storage/postgres/cross_signing_keys_table.go
index 4862d137..766e94b3 100644
--- a/keyserver/storage/postgres/cross_signing_keys_table.go
+++ b/keyserver/storage/postgres/cross_signing_keys_table.go
@@ -68,7 +68,6 @@ func (s *crossSigningKeysStatements) SelectCrossSigningKeysForUser(
 		return nil, err
 	}
 	defer internal.CloseAndLogIfError(ctx, rows, "selectCrossSigningKeysForUserStmt: rows.close() failed")
-	r = api.CrossSigningKeyMap{}
 	for rows.Next() {
 		var keyType gomatrixserverlib.CrossSigningKeyPurpose
 		var keyID gomatrixserverlib.KeyID
@@ -76,6 +75,9 @@ func (s *crossSigningKeysStatements) SelectCrossSigningKeysForUser(
 		if err := rows.Scan(&keyType, &keyID, &keyData); err != nil {
 			return nil, err
 		}
+		if _, ok := r[keyType]; !ok {
+			r[keyType] = map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{}
+		}
 		r[keyType][keyID] = keyData
 	}
 	return
diff --git a/keyserver/storage/sqlite3/cross_signing_keys_table.go b/keyserver/storage/sqlite3/cross_signing_keys_table.go
index 76092b02..d647906d 100644
--- a/keyserver/storage/sqlite3/cross_signing_keys_table.go
+++ b/keyserver/storage/sqlite3/cross_signing_keys_table.go
@@ -68,7 +68,6 @@ func (s *crossSigningKeysStatements) SelectCrossSigningKeysForUser(
 		return nil, err
 	}
 	defer internal.CloseAndLogIfError(ctx, rows, "selectCrossSigningKeysForUserStmt: rows.close() failed")
-	r = api.CrossSigningKeyMap{}
 	for rows.Next() {
 		var keyType gomatrixserverlib.CrossSigningKeyPurpose
 		var keyID gomatrixserverlib.KeyID
@@ -76,6 +75,9 @@ func (s *crossSigningKeysStatements) SelectCrossSigningKeysForUser(
 		if err := rows.Scan(&keyType, &keyID, &keyData); err != nil {
 			return nil, err
 		}
+		if _, ok := r[keyType]; !ok {
+			r[keyType] = map[gomatrixserverlib.KeyID]gomatrixserverlib.Base64Bytes{}
+		}
 		r[keyType][keyID] = keyData
 	}
 	return