diff --git a/Source/Additions/GSObjCRuntime.m b/Source/Additions/GSObjCRuntime.m
index fa644cb7e..3bd0422dd 100644
--- a/Source/Additions/GSObjCRuntime.m
+++ b/Source/Additions/GSObjCRuntime.m
@@ -169,6 +169,9 @@ objc_sizeOfType(const char *type)
 	break;
 
       case _C_BFLD:
+#if	!defined(BITS_PER_UNIT)
+#  define	BITS_PER_UNIT	(sizeof(int)*8)
+#endif
 	{
 	  /* The new encoding of bitfields is: b 'position' 'type' 'size' */
 	  int position, size;