diff --git a/ChangeLog b/ChangeLog
index 2839b1b31..eaa6160a2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2015-07-22  Richard Frith-Macdonald <rfm@gnu.org>
+
+	* Source/NSConcreteHashTable.m:
+	* Source/NSConcreteMapTable.m:
+	Get instance size at run time, so we can compile using nonfragile-abi
+
 2015-07-16 Niels Grewe <niels.grewe@halbordnung.de>
 
 	* Source/GSSet.m
diff --git a/Source/NSConcreteHashTable.m b/Source/NSConcreteHashTable.m
index d4f5eb6f6..74fd9f4e7 100644
--- a/Source/NSConcreteHashTable.m
+++ b/Source/NSConcreteHashTable.m
@@ -827,11 +827,14 @@ const NSHashTableCallBacks NSPointerToStructHashCallBacks =
 
 @implementation	NSConcreteHashTable
 
+static unsigned instanceSize = 0;
+
 + (void) initialize
 {
   if (concreteClass == Nil)
     {
       concreteClass = [NSConcreteHashTable class];
+      instanceSize = class_getInstanceSize(concreteClass);
     }
 #if	GS_WITH_GC
   /* We create a typed memory descriptor for hash nodes.
@@ -1084,7 +1087,7 @@ const NSHashTableCallBacks NSPointerToStructHashCallBacks =
  *      }
  *    GSIMapEndEnumerator(&enumerator);
  */
-      size += GSIMapSize(self) - sizeof(GSI_MAP_TABLE_T);
+      size += GSIMapSize(self) - instanceSize;
     }
   return size;
 }
diff --git a/Source/NSConcreteMapTable.m b/Source/NSConcreteMapTable.m
index d44680f26..3feb37358 100644
--- a/Source/NSConcreteMapTable.m
+++ b/Source/NSConcreteMapTable.m
@@ -1185,11 +1185,14 @@ const NSMapTableValueCallBacks NSOwnedPointerMapValueCallBacks =
 
 @implementation	NSConcreteMapTable
 
+static unsigned instanceSize = 0;
+
 + (void) initialize
 {
   if (concreteClass == Nil)
     {
       concreteClass = [NSConcreteMapTable class];
+      instanceSize = class_getInstanceSize(concreteClass);
     }
 #if	GS_WITH_GC
   /* We create a typed memory descriptor for map nodes.
@@ -1450,7 +1453,7 @@ const NSMapTableValueCallBacks NSOwnedPointerMapValueCallBacks =
  *      }
  *    GSIMapEndEnumerator(&enumerator);
  */
-      size += GSIMapSize(self) - sizeof(GSI_MAP_TABLE_T);
+      size += GSIMapSize(self) - instanceSize;
     }
   return size;
 }