diff --git a/source/api_c/include/c_api.h b/source/api_c/include/c_api.h
index e571547acc..b0c030962a 100644
--- a/source/api_c/include/c_api.h
+++ b/source/api_c/include/c_api.h
@@ -2,6 +2,9 @@
 #pragma once
 #ifdef __cplusplus
 extern "C" {
+#else
+// for C99
+#include <stdbool.h>
 #endif
 
 /**