diff --git a/biology/py-biopython/Makefile b/biology/py-biopython/Makefile
index 856f7d776e5..61cec9a6853 100644
--- a/biology/py-biopython/Makefile
+++ b/biology/py-biopython/Makefile
@@ -41,4 +41,9 @@ do-test:
 		PYTHONPATH=${WRKINST}${MODPY_SITEPKG} \
 		${MODPY_BIN} ./run_tests.py --offline
 
+.include <bsd.port.arch.mk>
+.if !${PROPERTIES:Mclang}
+CFLAGS +=	-std=gnu99
+.endif
+
 .include <bsd.port.mk>