diff --git a/docs/spec/04.00.00.conventions.md b/docs/spec/04.00.00.conventions.md
index cf2268f..687bca6 100644
--- a/docs/spec/04.00.00.conventions.md
+++ b/docs/spec/04.00.00.conventions.md
@@ -78,6 +78,8 @@ section is the header. The second section contains the metadata. This section is
   * E.g. sz = buffer_size                              varUI32
   * buffer                                             UI8[sz]
 
+* All uninitialized elements of opposite_corners_ shall be set to kInvalidCornerIndex.
+
 ### General Conventions