diff --git a/MAKE/Makefile.lumi_gnu_2403 b/MAKE/Makefile.lumi_gnu_2403
index c65be4cae..db31975e7 100644
--- a/MAKE/Makefile.lumi_gnu_2403
+++ b/MAKE/Makefile.lumi_gnu_2403
@@ -24,7 +24,7 @@ FLAGS =
 #GNU flags:
 CC_BRAND = gcc
 CC_BRAND_VERSION = 9.3.0
-CXXFLAGS += -g -O3 -fopenmp -funroll-loops -std=c++17 -W -Wall -Wno-unused -mfma -mavx2
+CXXFLAGS += -g -O3 -fopenmp -funroll-loops -std=c++20 -W -Wall -Wno-unused -mfma -mavx2
 testpackage: CXXFLAGS = -g -O2 -fopenmp -funroll-loops -std=c++20 -mno-fma -mno-avx2 -mno-avx
 
 MATHFLAGS = -ffast-math -fno-finite-math-only