diff --git a/mongoose.h b/mongoose.h
index 1b86bd57..213bd8b4 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -21,6 +21,8 @@
 #ifndef MONGOOSE_HEADER_INCLUDED
 #define  MONGOOSE_HEADER_INCLUDED
 
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif // __cplusplus