diff --git a/src/settings/plugins/ifupdown/parser.c b/src/settings/plugins/ifupdown/parser.c
index 350a62caec..d81fd84d85 100644
--- a/src/settings/plugins/ifupdown/parser.c
+++ b/src/settings/plugins/ifupdown/parser.c
@@ -36,6 +36,7 @@
 #include <nm-setting-8021x.h>
 #include <nm-system-config-interface.h>
 #include <nm-utils.h>
+#include <ctype.h>
 
 #include "parser.h"
 #include "plugin.h"