diff --git a/src/interfaces/ecpg/compatlib/informix.c b/src/interfaces/ecpg/compatlib/informix.c
index 23e0a820f2f..e32c09077c2 100644
--- a/src/interfaces/ecpg/compatlib/informix.c
+++ b/src/interfaces/ecpg/compatlib/informix.c
@@ -1,6 +1,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <ctype.h>
 
 #include <ecpg_informix.h>
 #include <pgtypes_error.h>