diff --git a/external/unbound/config.h.cmake.in b/external/unbound/config.h.cmake.in
index c64c3bab..861020ec 100644
--- a/external/unbound/config.h.cmake.in
+++ b/external/unbound/config.h.cmake.in
@@ -216,7 +216,7 @@
 #cmakedefine HAVE_IPHLPAPI_H
 
 /* Define to 1 if you have the `isblank' function. */
-#undef HAVE_ISBLANK
+#cmakedefine HAVE_ISBLANK
 
 /* Define to 1 if you have the `kill' function. */
 #cmakedefine HAVE_KILL