diff --git a/lib/checkother.cpp b/lib/checkother.cpp
index a18c4f479..1d0b30b70 100644
--- a/lib/checkother.cpp
+++ b/lib/checkother.cpp
@@ -21,6 +21,7 @@
 #include "checkother.h"
 #include "mathlib.h"
 
+#include <cctype> // std::isupper
 #include <cmath> // fabs()
 //---------------------------------------------------------------------------