diff --git a/src/strigger/opts.c b/src/strigger/opts.c
index 462dc70fdceb7447a0141fffb07def9580281bb1..dbfb3989c85821433e0b72c3731fa4aa0f582440 100644
--- a/src/strigger/opts.c
+++ b/src/strigger/opts.c
@@ -45,6 +45,7 @@
 #  define _GNU_SOURCE
 #endif
 
+#include <ctype.h>
 #include <getopt.h>
 #include <limits.h>
 #include <pwd.h>