diff --git a/include/bits/ctype.h b/include/bits/ctype.h
new file mode 100644
index 0000000000000000000000000000000000000000..e5dedf5d304a1da9724e6e27226cda1dfa75c35d
--- /dev/null
+++ b/include/bits/ctype.h
@@ -0,0 +1,7 @@
+#ifndef _BITS_CTYPE_H
+#define _BITS_CTYPE_H
+
+#define _tolower(c) tolower(c)
+#define _toupper(c) toupper(c)
+
+#endif /* _BITS_CTYPE_H */
diff --git a/src/header/ctype/cbindgen.toml b/src/header/ctype/cbindgen.toml
index 4fadb8447717c3292b035e2431bb369d23550cf6..eb9b10d151ba60ce40118f1d63658e44f093a233 100644
--- a/src/header/ctype/cbindgen.toml
+++ b/src/header/ctype/cbindgen.toml
@@ -1,4 +1,4 @@
-sys_includes = []
+sys_includes = ["bits/ctype.h"]
 include_guard = "_CTYPE_H"
 language = "C"
 style = "Tag"