diff --git a/ivar.c b/ivar.c
index 671eea4..37f99d7 100644
--- a/ivar.c
+++ b/ivar.c
@@ -22,6 +22,11 @@ PRIVATE void objc_compute_ivar_offsets(Class class)
 		legacy = (struct objc_ivar_list_legacy *)class->ivars;
 		class->ivars = upgradeIvarList(class, legacy);
 	}
+	if (class->ivars->size != sizeof(struct objc_ivar))
+	{
+		fprintf(stderr, "Downgrading ivar struct not yet implemented");
+		abort();
+	}
 	int i = 0;
 	/* If this class was compiled with support for late-bound ivars, the
 	* instance_size field will contain 0 - {the size of the instance variables
@@ -159,6 +164,7 @@ static struct objc_ivar_list *upgradeIvarList(Class cls, struct objc_ivar_list_l
 	}
 	struct objc_ivar_list *n = calloc(1, sizeof(struct objc_ivar_list) +
 			l->count*sizeof(struct objc_ivar));
+	n->size = sizeof(struct objc_ivar);
 	n->count = l->count;
 	for (int i=0 ; i<l->count ; i++)
 	{
diff --git a/ivar.h b/ivar.h
index a7b42bb..5ebcf20 100644
--- a/ivar.h
+++ b/ivar.h
@@ -27,7 +27,7 @@ struct objc_ivar
 	/**
 	 * Flags for this instance variable.
 	 */
-	int         flags;
+	long        flags;
 };
 
 /**
@@ -107,6 +107,12 @@ struct objc_ivar_list
 	 * The number of instance variables in this list.
 	 */
 	int              count;
+	/**
+	 * The size of a `struct objc_ivar`.  This allows the runtime to load
+	 * versions of this that come from a newer compiler, if we ever need to do
+	 * so.
+	 */
+	size_t           size;
 	/**
 	 * An array of instance variable metadata structures.  Note that this array
 	 * has count elements.
diff --git a/runtime.c b/runtime.c
index c52dadf..4c56a29 100644
--- a/runtime.c
+++ b/runtime.c
@@ -129,6 +129,7 @@ BOOL class_addIvar(Class cls, const char *name, size_t size, uint8_t alignment,
 	if (NULL == ivarlist)
 	{
 		cls->ivars = malloc(sizeof(struct objc_ivar_list) + sizeof(struct objc_ivar));
+		cls->ivars->size = sizeof(struct objc_ivar);
 		cls->ivars->count = 1;
 	}
 	else