diff --git a/block.h b/block.h
index 283ba69b..6eeae33f 100644
--- a/block.h
+++ b/block.h
@@ -35,9 +35,9 @@
  * Structure of Block options. 
  */
 typedef struct {
-  unsigned int num:20;		/**< block number */
-  unsigned int m:1;		/**< 1 if more blocks follow, 0 otherwise */
-  unsigned int szx:3;		/**< block size */
+  unsigned int num;	  /**< block number */
+  uint8_t m:1;		  /**< 1 if more blocks follow, 0 otherwise */
+  uint8_t szx:3;	  /**< block size */
 } coap_block_t;
 
 /**