diff --git a/src/coreboot/default.nix b/src/coreboot/default.nix
index bbe2f8f..fff3c39 100644
--- a/src/coreboot/default.nix
+++ b/src/coreboot/default.nix
@@ -186,7 +186,7 @@ stdenv.mkDerivation {
           "COREBOOT_ROMSIZE_KB_${toString flash-chip-size-in-kbytes}" = lib.mkForce yes;
 
           # TODO: the following two options might be unnecessary
-          ROM_SIZE  = lib.mkForce (freeform "0x${lib.toHexString flash-chip-size-in-bytes}");
+          ROM_SIZE  = lib.mkForce (freeform "0x${lib.toUpper (lib.toHexString flash-chip-size-in-bytes)}");
           COREBOOT_ROMSIZE_KB = lib.mkForce (freeform (toString flash-chip-size-in-kbytes));
         } // lib.optionalAttrs (uart-for-console != null) {
           UART_FOR_CONSOLE    = lib.mkForce (freeform "${builtins.toString uart-for-console}");