diff --git a/pkg/umorse/Makefile b/pkg/umorse/Makefile
index 919d1a4703193f6c9babe0244128119a5f6f5aa0..394f1d46e0838cdd97ef1418d554ce85d0dd699c 100644
--- a/pkg/umorse/Makefile
+++ b/pkg/umorse/Makefile
@@ -3,6 +3,8 @@ PKG_URL=https://github.com/smlng/uMorse
 PKG_VERSION=1dc14abdba22cca2f7efc053b2bce327bc7db97e
 PKG_LICENSE=MIT
 
+CFLAGS += -D_XOPEN_SOURCE=600
+
 .PHONY: all
 
 all: git-download