diff --git a/payload_constants.h b/payload_constants.h
index 81bc36a..38cc075 100644
--- a/payload_constants.h
+++ b/payload_constants.h
@@ -48,7 +48,7 @@
 extern const char kLegacyPartitionNameRoot[];
 
 extern const char kBspatchPath[];
-extern const char kDeltaMagic[];
+extern const char kDeltaMagic[4];
 
 // A block number denoting a hole on a sparse file. Used on Extents to refer to
 // section of blocks not present on disk on a sparse file.