diff --git a/tools/cpp/src/cpp-build/generate_geocoding_data.cc b/tools/cpp/src/cpp-build/generate_geocoding_data.cc
index ef94ef674..546a7e9ac 100644
--- a/tools/cpp/src/cpp-build/generate_geocoding_data.cc
+++ b/tools/cpp/src/cpp-build/generate_geocoding_data.cc
@@ -20,6 +20,7 @@
 #include <locale>
 #include <sys/stat.h>
 #include <algorithm>
+#include <cctype>
 #include <cmath>
 #include <cstdio>
 #include <cstring>