diff --git a/src/factory/WFTaskFactory.h b/src/factory/WFTaskFactory.h
index 52fe9305..95fa6476 100644
--- a/src/factory/WFTaskFactory.h
+++ b/src/factory/WFTaskFactory.h
@@ -23,7 +23,6 @@
 #include <sys/types.h>
 #include <sys/uio.h>
 #include <time.h>
-#include <stddef.h>
 #include <stdint.h>
 #include <utility>
 #include <functional>
@@ -424,6 +423,10 @@ public:
 		return task;
 	}
 
+private:
+	/* Some compilers don't declare 'nullptr_t' although required by C++11. */
+	using nullptr_t = std::nullptr_t;
+
 public:
 	/* The following functions are for overload resolution only. */
 
diff --git a/src/factory/WFTaskFactory.inl b/src/factory/WFTaskFactory.inl
index ea84f4f1..c88c3563 100644
--- a/src/factory/WFTaskFactory.inl
+++ b/src/factory/WFTaskFactory.inl
@@ -23,7 +23,6 @@
 #include <errno.h>
 #include <time.h>
 #include <netdb.h>
-#include <stddef.h>
 #include <stdio.h>
 #include <string>
 #include <functional>