diff --git a/libs/freetdm/Makefile.am b/libs/freetdm/Makefile.am
index 7370ae2c8d..d96c3f1963 100644
--- a/libs/freetdm/Makefile.am
+++ b/libs/freetdm/Makefile.am
@@ -251,7 +251,7 @@ endif
 if HAVE_MISDN
 mod_LTLIBRARIES += ftmod_misdn.la
 ftmod_misdn_la_SOURCES  = $(SRC)/ftmod/ftmod_misdn/ftmod_misdn.c
-ftmod_misdn_la_CFLAGS   = $(FTDM_CFLAGS) $(AM_CFLAGS) $(MISDN_CFLAGS) -D_BSD_SOURCE
+ftmod_misdn_la_CFLAGS   = $(FTDM_CFLAGS) $(AM_CFLAGS) $(MISDN_CFLAGS) -D_GNU_SOURCE
 ftmod_misdn_la_LDFLAGS  = -shared -module -avoid-version
 ftmod_misdn_la_LIBADD   = libfreetdm.la
 endif