diff --git a/lib/utils/buffer_helper.c b/lib/utils/buffer_helper.c
index f6eb8bf1e7..694952e3d2 100644
--- a/lib/utils/buffer_helper.c
+++ b/lib/utils/buffer_helper.c
@@ -26,10 +26,10 @@
 
 #include "lib/utils/buffer_helper.h"
 
-void normalize_buffer_bounds(int32_t* start, int32_t end, uint32_t* length) {
+void normalize_buffer_bounds(int32_t* start, int32_t end, size_t* length) {
     if (end < 0) {
         end += *length;
-    } else if (((uint32_t) end) > *length) {
+    } else if (((size_t) end) > *length) {
         end = *length;
     }
     if (*start < 0) {
diff --git a/lib/utils/buffer_helper.h b/lib/utils/buffer_helper.h
index 47042e7ef1..7487b9df53 100644
--- a/lib/utils/buffer_helper.h
+++ b/lib/utils/buffer_helper.h
@@ -28,7 +28,8 @@
 #define MICROPY_INCLUDED_LIB_UTILS_BUFFER_HELPER_H
 
 #include <stdint.h>
+#include <string.h>
 
-void normalize_buffer_bounds(int32_t* start, int32_t end, uint32_t* length);
+void normalize_buffer_bounds(int32_t* start, int32_t end, size_t* length);
 
 #endif  // MICROPY_INCLUDED_LIB_UTILS_BUFFER_HELPER_H
diff --git a/shared-bindings/bitbangio/SPI.c b/shared-bindings/bitbangio/SPI.c
index 88bd3d6cb3..9a51bde665 100644
--- a/shared-bindings/bitbangio/SPI.c
+++ b/shared-bindings/bitbangio/SPI.c
@@ -275,13 +275,13 @@ STATIC mp_obj_t bitbangio_spi_write_readinto(size_t n_args, const mp_obj_t *pos_
     mp_buffer_info_t buf_out_info;
     mp_get_buffer_raise(args[ARG_buffer_out].u_obj, &buf_out_info, MP_BUFFER_READ);
     int32_t out_start = args[ARG_out_start].u_int;
-    uint32_t out_length = buf_out_info.len;
+    size_t out_length = buf_out_info.len;
     normalize_buffer_bounds(&out_start, args[ARG_out_end].u_int, &out_length);
 
     mp_buffer_info_t buf_in_info;
     mp_get_buffer_raise(args[ARG_buffer_in].u_obj, &buf_in_info, MP_BUFFER_WRITE);
     int32_t in_start = args[ARG_in_start].u_int;
-    uint32_t in_length = buf_in_info.len;
+    size_t in_length = buf_in_info.len;
     normalize_buffer_bounds(&in_start, args[ARG_in_end].u_int, &in_length);
 
     if (out_length != in_length) {
diff --git a/shared-bindings/busio/SPI.c b/shared-bindings/busio/SPI.c
index a7d7d515c5..d47bf499a2 100644
--- a/shared-bindings/busio/SPI.c
+++ b/shared-bindings/busio/SPI.c
@@ -244,7 +244,7 @@ STATIC mp_obj_t busio_spi_write(size_t n_args, const mp_obj_t *pos_args, mp_map_
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[ARG_buffer].u_obj, &bufinfo, MP_BUFFER_READ);
     int32_t start = args[ARG_start].u_int;
-    uint32_t length = bufinfo.len;
+    size_t length = bufinfo.len;
     normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
 
     if (length == 0) {
@@ -288,7 +288,7 @@ STATIC mp_obj_t busio_spi_readinto(size_t n_args, const mp_obj_t *pos_args, mp_m
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[ARG_buffer].u_obj, &bufinfo, MP_BUFFER_WRITE);
     int32_t start = args[ARG_start].u_int;
-    uint32_t length = bufinfo.len;
+    size_t length = bufinfo.len;
     normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
 
     if (length == 0) {
@@ -337,13 +337,13 @@ STATIC mp_obj_t busio_spi_write_readinto(size_t n_args, const mp_obj_t *pos_args
     mp_buffer_info_t buf_out_info;
     mp_get_buffer_raise(args[ARG_buffer_out].u_obj, &buf_out_info, MP_BUFFER_READ);
     int32_t out_start = args[ARG_out_start].u_int;
-    uint32_t out_length = buf_out_info.len;
+    size_t out_length = buf_out_info.len;
     normalize_buffer_bounds(&out_start, args[ARG_out_end].u_int, &out_length);
 
     mp_buffer_info_t buf_in_info;
     mp_get_buffer_raise(args[ARG_buffer_in].u_obj, &buf_in_info, MP_BUFFER_WRITE);
     int32_t in_start = args[ARG_in_start].u_int;
-    uint32_t in_length = buf_in_info.len;
+    size_t in_length = buf_in_info.len;
     normalize_buffer_bounds(&in_start, args[ARG_in_end].u_int, &in_length);
 
     if (out_length != in_length) {