diff --git a/library/ecdsa.c b/library/ecdsa.c
index 630d5bd..18f4379 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -685,7 +685,9 @@
     ECDSA_VALIDATE_RET( hash  != NULL );
     ECDSA_VALIDATE_RET( sig   != NULL );
     ECDSA_VALIDATE_RET( slen  != NULL );
-    ECDSA_VALIDATE_RET( f_rng != NULL );
+
+    if( f_rng == NULL )
+        return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
 
     mbedtls_mpi_init( &r );
     mbedtls_mpi_init( &s );