diff --git a/src/toolsComm/flex/flexdef.h b/src/toolsComm/flex/flexdef.h
index 76bf2afc8..df009a281 100644
--- a/src/toolsComm/flex/flexdef.h
+++ b/src/toolsComm/flex/flexdef.h
@@ -43,6 +43,7 @@
 #include <string.h>
 #include <stddef.h>
 #include <stdlib.h>
+#include <ctype.h>
 
 #ifdef __GNUC__
 #define NORETURN __attribute__((noreturn))