diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index c562c0c0434..e872bf6a313 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -2460,7 +2460,7 @@ CreateSharedBackendStatus(void)
 
 	if (!found)
 	{
-		MemSet(BackendActivityBuffer, 0, size);
+		MemSet(BackendActivityBuffer, 0, BackendActivityBufferSize);
 
 		/* Initialize st_activity pointers. */
 		buffer = BackendActivityBuffer;