diff --git a/src/matrix-types.c b/src/matrix-types.c
index e7a45d3..9f40561 100644
--- a/src/matrix-types.c
+++ b/src/matrix-types.c
@@ -159,6 +159,7 @@ G_DEFINE_QUARK(matrix-error-quark, matrix_error);
 
 /**
  * MatrixReceiptType:
+ * @MATRIX_RECEIPT_TYPE_UNKNOWN: the receipt type is unknown to this SDK
  * @MATRIX_RECEIPT_TYPE_READ: indicate that the message has been read
  *
  * Receipt types of acknowledgment.
diff --git a/src/matrix-types.h b/src/matrix-types.h
index 101f520..151bebc 100644
--- a/src/matrix-types.h
+++ b/src/matrix-types.h
@@ -116,6 +116,7 @@ typedef enum {
 } MatrixPusherKind;
 
 typedef enum {
+    MATRIX_RECEIPT_TYPE_UNKNOWN,
     MATRIX_RECEIPT_TYPE_READ
 } MatrixReceiptType;