diff --git a/library/hmac_drbg.c b/library/hmac_drbg.c
index edecc6e..67123df 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -74,7 +74,7 @@
     unsigned char rounds = ( additional != NULL && add_len != 0 ) ? 2 : 1;
     unsigned char sep[1];
     unsigned char K[MBEDTLS_MD_MAX_SIZE];
-    int ret = 0;
+    int ret = MBEDTLS_ERR_MD_BAD_INPUT_DATA;
 
     for( sep[0] = 0; sep[0] < rounds; sep[0]++ )
     {