diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 5f541ad..d477fc1 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -5222,7 +5222,7 @@
 
     /* Helper references for top part of N */
     mbedtls_mpi_uint * const NT_p = N->p + P255_WIDTH;
-    unsigned const NT_n = N->n - P255_WIDTH;
+    size_t const NT_n = N->n - P255_WIDTH;
     if( NT_n == 0 || NT_n > P255_WIDTH )
         return( 0 );