diff --git a/mongoose.h b/mongoose.h
index 1b86bd5789455ec8a47e6b9d97a20db5d9fc2ba0..213bd8b466b8c1f750120c70c89c4b75613525bd 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