diff --git a/lib/bbitset.h b/lib/bbitset.h
index 3accc74c..7dd62f96 100644
--- a/lib/bbitset.h
+++ b/lib/bbitset.h
@@ -23,6 +23,7 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
 
 #include <stdbool.h>
 #include <limits.h>
+#include <stddef.h>
 
 /* Currently we support five flavours of bitsets:
    BITSET_ARRAY:  Array of bits (fixed size, fast for dense bitsets).