diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 1808152..2ad14e1 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -5435,7 +5435,7 @@
     if (X_Limbs > 2*P255_WIDTH) {
         X_Limbs = 2*P255_WIDTH;
     } else if (X_Limbs < P255_WIDTH) {
-        return 0;
+        return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
     }
 
     mbedtls_mpi_uint *carry = mbedtls_calloc(P255_WIDTH, ciL);