diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index 8ecab4e..9824146 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -334,9 +334,13 @@
  *                  through (will be updated in case of a new entry).
  * \param oid       The OID to look for.
  * \param oid_len   The size of the OID.
- * \param val       The data to store (can be \c NULL if you want to fill
- *                  it by hand).
+ * \param val       The associated data to store. If this is \c NULL,
+ *                  no data is copied to the new or existing buffer.
  * \param val_len   The minimum length of the data buffer needed.
+ *                  If this is 0, do not allocate a buffer for the associated
+ *                  data.
+ *                  If the OID was already present, enlarge, shrink or free
+ *                  the existing buffer to fit \p val_len.
  *
  * \return          A pointer to the new / existing entry on success.
  * \return          \c NULL if if there was a memory allocation error.
diff --git a/library/asn1write.c b/library/asn1write.c
index 98c6766..a138d0b 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -432,18 +432,26 @@
         memcpy( cur->oid.p, oid, oid_len );
 
         cur->val.len = val_len;
-        cur->val.p = mbedtls_calloc( 1, val_len );
-        if( cur->val.p == NULL )
+        if( val_len != 0 )
         {
-            mbedtls_free( cur->oid.p );
-            mbedtls_free( cur );
-            return( NULL );
+            cur->val.p = mbedtls_calloc( 1, val_len );
+            if( cur->val.p == NULL )
+            {
+                mbedtls_free( cur->oid.p );
+                mbedtls_free( cur );
+                return( NULL );
+            }
         }
 
         cur->next = *head;
         *head = cur;
     }
-    else if( cur->val.len < val_len )
+    else if( val_len == 0 )
+    {
+        mbedtls_free( cur->val.p );
+        cur->val.p = NULL;
+    }
+    else if( cur->val.len != val_len )
     {
         /*
          * Enlarge existing value buffer if needed