diff --git a/block.c b/block.c
index 0c7027cd..847fe9cc 100644
--- a/block.c
+++ b/block.c
@@ -18,6 +18,25 @@
 #define min(a,b) ((a) < (b) ? (a) : (b))
 
 #ifndef WITHOUT_BLOCK
+unsigned int
+coap_opt_block_num(const coap_opt_t *block_opt) {
+  unsigned int num = 0;
+  unsigned short len;
+  
+  len = coap_opt_length(block_opt);
+
+  if (len == 0) {
+    return 0;
+  }
+  
+  if (len > 1) {
+    num = coap_decode_var_bytes(COAP_OPT_VALUE(block_opt), 
+				COAP_OPT_LENGTH(block_opt) - 1);
+  }
+  
+  return (num << 4) | ((*COAP_OPT_BLOCK_LAST(block_opt) & 0xF0) >> 4);
+}
+
 int
 coap_get_block(coap_pdu_t *pdu, unsigned short type, coap_block_t *block) {
   coap_opt_iterator_t opt_iter;
@@ -30,8 +49,7 @@ coap_get_block(coap_pdu_t *pdu, unsigned short type, coap_block_t *block) {
     block->szx = COAP_OPT_BLOCK_SZX(option);
     if (COAP_OPT_BLOCK_MORE(option))
       block->m = 1;
-    block->num = COAP_OPT_BLOCK_NUM(option);
-
+    block->num = coap_opt_block_num(option);
     return 1;
   }
 
diff --git a/block.h b/block.h
index 73c3656f..283ba69b 100644
--- a/block.h
+++ b/block.h
@@ -40,29 +40,27 @@ typedef struct {
   unsigned int szx:3;		/**< block size */
 } coap_block_t;
 
-/** Returns the value of the least significant byte of a Block option @p opt. */
-#define COAP_OPT_BLOCK_LAST(opt) ( COAP_OPT_VALUE(opt) + (COAP_OPT_LENGTH(opt) - 1) )
+/**
+ * Returns the value of the least significant byte of a Block option @p opt. 
+ * For zero-length options (i.e. num == m == szx == 0), COAP_OPT_BLOCK_LAST
+ * returns @c NULL.
+ */
+#define COAP_OPT_BLOCK_LAST(opt) \
+  (COAP_OPT_LENGTH(opt) ? (COAP_OPT_VALUE(opt) + (COAP_OPT_LENGTH(opt)-1)) : 0)
 
 /** Returns the value of the More-bit of a Block option @p opt. */
-#define COAP_OPT_BLOCK_MORE(opt) ( *COAP_OPT_BLOCK_LAST(opt) & 0x08 )
+#define COAP_OPT_BLOCK_MORE(opt) \
+  (COAP_OPT_LENGTH(opt) ? (*COAP_OPT_BLOCK_LAST(opt) & 0x08) : 0)
 
 /** Returns the value of the SZX-field of a Block option @p opt. */
-#define COAP_OPT_BLOCK_SZX(opt)  ( *COAP_OPT_BLOCK_LAST(opt) & 0x07 )
+#define COAP_OPT_BLOCK_SZX(opt)  \
+  (COAP_OPT_LENGTH(opt) ? (*COAP_OPT_BLOCK_LAST(opt) & 0x07) : 0)
 
-/** Implementation of COAP_OPT_BLOCK_NUM */
-static inline unsigned int
-_coap_block_num_impl(const coap_opt_t *block_opt) {
-  unsigned int num = 0;
-
-  if (COAP_OPT_LENGTH(block_opt) > 1)
-    num = coap_decode_var_bytes(COAP_OPT_VALUE(block_opt), 
-				COAP_OPT_LENGTH(block_opt) - 1);
-
-  return (num << 4) | ((*COAP_OPT_BLOCK_LAST(block_opt) & 0xF0) >> 4);
-}
-
-/** Returns the value of field @c num in the given block option @p Block. */
-#define COAP_OPT_BLOCK_NUM(Block) _coap_block_num_impl(Block)
+/**
+ * Returns the value of field @c num in the given block option @p
+ * block_opt.
+ */
+unsigned int coap_opt_block_num(const coap_opt_t *block_opt);
 
 /**
  * Checks if more than @p num blocks are required to deliver @p data_len