diff --git a/src/backend/utils/mmgr/generation.c b/src/backend/utils/mmgr/generation.c
index 4fb8663cd6b..8c2ca87705d 100644
--- a/src/backend/utils/mmgr/generation.c
+++ b/src/backend/utils/mmgr/generation.c
@@ -780,8 +780,8 @@ GenerationRealloc(void *pointer, Size size)
 #endif
 
 	/*
-	 * Maybe the allocated area already is >= the new size.  (In particular,
-	 * we always fall out here if the requested size is a decrease.)
+	 * Maybe the allocated area already big enough.  (In particular, we always
+	 * fall out here if the requested size is a decrease.)
 	 *
 	 * This memory context does not use power-of-2 chunk sizing and instead
 	 * carves the chunks to be as small as possible, so most repalloc() calls
@@ -789,7 +789,12 @@ GenerationRealloc(void *pointer, Size size)
 	 *
 	 * XXX Perhaps we should annotate this condition with unlikely()?
 	 */
+#ifdef MEMORY_CONTEXT_CHECKING
+	/* With MEMORY_CONTEXT_CHECKING, we need an extra byte for the sentinel */
+	if (oldsize > size)
+#else
 	if (oldsize >= size)
+#endif
 	{
 #ifdef MEMORY_CONTEXT_CHECKING
 		Size		oldrequest = chunk->requested_size;