diff --git a/src/flb_hash_table.c b/src/flb_hash_table.c
index ea63d4c8fd5..71fa109017f 100644
--- a/src/flb_hash_table.c
+++ b/src/flb_hash_table.c
@@ -18,6 +18,7 @@
  */
 
 #include <stdint.h>
+#include <ctype.h>
 
 #include <fluent-bit/flb_info.h>
 #include <fluent-bit/flb_mem.h>