diff --git a/blktemplate.h b/blktemplate.h
index f8fdd76..5efa089 100644
--- a/blktemplate.h
+++ b/blktemplate.h
@@ -90,6 +90,7 @@ extern gbt_capabilities_t blktmpl_getcapability(const char *);
 
 typedef gbt_capabilities_t blkmutations_t;
 
+// WARNING: Do not allocate this (ABI is not guaranteed to remain fixed-size)
 typedef struct {
 	uint32_t version;
 	unsigned char diffbits[4];