diff --git a/fio.h b/fio.h
index d2cca11..08f1733 100644
--- a/fio.h
+++ b/fio.h
@@ -531,7 +531,7 @@
 	assert(!(io_u->ddir == DDIR_WRITE && !td_write(td)));
 }
 
-#define BLOCKS_PER_MAP		(8 * sizeof(int))
+#define BLOCKS_PER_MAP		(8 * sizeof(unsigned long))
 #define TO_MAP_BLOCK(f, b)	(b)
 #define RAND_MAP_IDX(f, b)	(TO_MAP_BLOCK(f, b) / BLOCKS_PER_MAP)
 #define RAND_MAP_BIT(f, b)	(TO_MAP_BLOCK(f, b) & (BLOCKS_PER_MAP - 1))