diff --git a/libc/include/wchar.h b/libc/include/wchar.h
index 89c6fb6..04818b9 100644
--- a/libc/include/wchar.h
+++ b/libc/include/wchar.h
@@ -70,6 +70,7 @@
 extern int               fwscanf(FILE *, const wchar_t *, ...);
 extern int               iswalnum(wint_t);
 extern int               iswalpha(wint_t);
+extern int               iswblank(wint_t);
 extern int               iswcntrl(wint_t);
 extern int               iswdigit(wint_t);
 extern int               iswgraph(wint_t);