diff --git a/library/ecdsa.c b/library/ecdsa.c
index 4156f3c..8892317 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -81,6 +81,10 @@
     if( grp->N.p == NULL )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
 
+    /* Make sure d is in range 1..n-1 */
+    if( mbedtls_mpi_cmp_int( d, 1 ) < 0 || mbedtls_mpi_cmp_mpi( d, &grp->N ) >= 0 )
+        return( MBEDTLS_ERR_ECP_INVALID_KEY );
+
     mbedtls_ecp_point_init( &R );
     mbedtls_mpi_init( &k ); mbedtls_mpi_init( &e ); mbedtls_mpi_init( &t );