diff --git a/apis/c/operator/operator_api.h b/apis/c/operator/operator_api.h
index 5ba25954..9118e98d 100644
--- a/apis/c/operator/operator_api.h
+++ b/apis/c/operator/operator_api.h
@@ -1,16 +1,18 @@
+#include <stddef.h>
+
 int dora_init_operator(void **operator_context);
 
 void dora_drop_operator(void *operator_context);
 
 int dora_on_input(
     const char *id_start,
-    unsigned int id_len,
+    size_t id_len,
     const char *data_start,
-    unsigned int data_len,
+    size_t data_len,
     const int (*output_fn_raw)(const char *id_start,
-                               unsigned int id_len,
+                               size_t id_len,
                                const char *data_start,
-                               unsigned int data_len,
+                               size_t data_len,
                                const void *output_context),
     void *output_context,
     const void *operator_context);