diff --git a/library/ecdh.c b/library/ecdh.c
index 302531d..ebc1676 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -246,7 +246,7 @@
     if( mpi_size( &ctx->z ) > blen )
         return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
 
-    *olen = ctx->grp.nbits / 8 + ( ( ctx->grp.nbits % 8 ) != 0 );
+    *olen = ctx->grp.pbits / 8 + ( ( ctx->grp.pbits % 8 ) != 0 );
     return mpi_write_binary( &ctx->z, buf, *olen );
 }