diff --git a/include/efi_api.h b/include/efi_api.h
index aa4306a..c44dc9d 100644
--- a/include/efi_api.h
+++ b/include/efi_api.h
@@ -71,7 +71,7 @@ struct efi_boot_services {
 					 enum efi_timer_delay type,
 					 uint64_t trigger_time);
 	efi_status_t (EFIAPI *wait_for_event)(unsigned long number_of_events,
-			struct efi_event **event, unsigned long *index);
+			struct efi_event **event, size_t *index);
 	efi_status_t (EFIAPI *signal_event)(struct efi_event *event);
 	efi_status_t (EFIAPI *close_event)(struct efi_event *event);
 	efi_status_t (EFIAPI *check_event)(struct efi_event *event);
diff --git a/lib/efi_loader/efi_boottime.c b/lib/efi_loader/efi_boottime.c
index e5adc17..976d582 100644
--- a/lib/efi_loader/efi_boottime.c
+++ b/lib/efi_loader/efi_boottime.c
@@ -553,7 +553,7 @@ static efi_status_t EFIAPI efi_set_timer_ext(struct efi_event *event,
  */
 static efi_status_t EFIAPI efi_wait_for_event(unsigned long num_events,
 					      struct efi_event **event,
-					      unsigned long *index)
+					      size_t *index)
 {
 	int i, j;
 
diff --git a/lib/efi_selftest/efi_selftest_events.c b/lib/efi_selftest/efi_selftest_events.c
index 532f165..b2cdc15 100644
--- a/lib/efi_selftest/efi_selftest_events.c
+++ b/lib/efi_selftest/efi_selftest_events.c
@@ -108,7 +108,7 @@ static int teardown(void)
  */
 static int execute(void)
 {
-	unsigned long index;
+	size_t index;
 	efi_status_t ret;
 
 	/* Set 10 ms timer */
diff --git a/lib/efi_selftest/efi_selftest_tpl.c b/lib/efi_selftest/efi_selftest_tpl.c
index 5d13f3b..0b78ee7 100644
--- a/lib/efi_selftest/efi_selftest_tpl.c
+++ b/lib/efi_selftest/efi_selftest_tpl.c
@@ -110,7 +110,7 @@ static int teardown(void)
  */
 static int execute(void)
 {
-	unsigned long index;
+	size_t index;
 	efi_status_t ret;
 	UINTN old_tpl;