diff --git a/ChangeLog b/ChangeLog
index 8888f994b9..5545e5d132 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -48,6 +48,9 @@ Bugfix
    * Fix ssl_client2 example to send application data with 0-length content
      when the request_size argument is set to 0 as stated in the documentation.
      Fixes #1833.
+   * Change the default behaviour of mbedtls_hkdf_extract() to return an error
+     when calling with a NULL salt and non-zero salt_len. Contributed by
+     Brian J Murray
 
 Changes
    * Change the shebang line in Perl scripts to look up perl in the PATH.
diff --git a/library/hkdf.c b/library/hkdf.c
index d2e55e869b..82d8a429f4 100644
--- a/library/hkdf.c
+++ b/library/hkdf.c
@@ -62,6 +62,11 @@ int mbedtls_hkdf_extract( const mbedtls_md_info_t *md,
     {
         size_t hash_len;
 
+        if( salt_len != 0 )
+        {
+            return MBEDTLS_ERR_HKDF_BAD_INPUT_DATA;
+        }
+
         hash_len = mbedtls_md_get_size( md );
 
         if( hash_len == 0 )
@@ -114,6 +119,10 @@ int mbedtls_hkdf_expand( const mbedtls_md_info_t *md, const unsigned char *prk,
         n++;
     }
 
+    /*
+     * Per RFC 5869 Section 2.3, okm_len must not exceed
+     * 255 times the hash length
+     */
     if( n > 255 )
     {
         return( MBEDTLS_ERR_HKDF_BAD_INPUT_DATA );
@@ -126,7 +135,10 @@ int mbedtls_hkdf_expand( const mbedtls_md_info_t *md, const unsigned char *prk,
         goto exit;
     }
 
-    /* RFC 5869 Section 2.3. */
+    /*
+     * Compute T = T(1) | T(2) | T(3) | ... | T(N)
+     * Where T(N) is defined in RFC 5869 Section 2.3
+     */
     for( i = 1; i <= n; i++ )
     {
         size_t num_to_copy;
@@ -150,7 +162,7 @@ int mbedtls_hkdf_expand( const mbedtls_md_info_t *md, const unsigned char *prk,
             goto exit;
         }
 
-        /* The constant concatenated to the end of each t(n) is a single octet.
+        /* The constant concatenated to the end of each T(n) is a single octet.
          * */
         ret = mbedtls_md_hmac_update( &ctx, &c, 1 );
         if( ret != 0 )