diff --git a/py/stream.c b/py/stream.c
index 448de41bbb..2a9acdea77 100644
--- a/py/stream.c
+++ b/py/stream.c
@@ -507,10 +507,10 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_ioctl_obj, 2, 3, stream_ioctl);
 // status, this variable will contain error no.
 int mp_stream_errno;
 
-ssize_t mp_stream_posix_write(mp_obj_t stream, const void *buf, size_t len) {
-    mp_obj_base_t* o = (mp_obj_base_t*)MP_OBJ_TO_PTR(stream);
+ssize_t mp_stream_posix_write(void *stream, const void *buf, size_t len) {
+    mp_obj_base_t* o = stream;
     const mp_stream_p_t *stream_p = o->type->protocol;
-    mp_uint_t out_sz = stream_p->write(stream, buf, len, &mp_stream_errno);
+    mp_uint_t out_sz = stream_p->write(MP_OBJ_FROM_PTR(stream), buf, len, &mp_stream_errno);
     if (out_sz == MP_STREAM_ERROR) {
         return -1;
     } else {
@@ -518,10 +518,10 @@ ssize_t mp_stream_posix_write(mp_obj_t stream, const void *buf, size_t len) {
     }
 }
 
-ssize_t mp_stream_posix_read(mp_obj_t stream, void *buf, size_t len) {
-    mp_obj_base_t* o = (mp_obj_base_t*)MP_OBJ_TO_PTR(stream);
+ssize_t mp_stream_posix_read(void *stream, void *buf, size_t len) {
+    mp_obj_base_t* o = stream;
     const mp_stream_p_t *stream_p = o->type->protocol;
-    mp_uint_t out_sz = stream_p->read(stream, buf, len, &mp_stream_errno);
+    mp_uint_t out_sz = stream_p->read(MP_OBJ_FROM_PTR(stream), buf, len, &mp_stream_errno);
     if (out_sz == MP_STREAM_ERROR) {
         return -1;
     } else {
@@ -529,23 +529,23 @@ ssize_t mp_stream_posix_read(mp_obj_t stream, void *buf, size_t len) {
     }
 }
 
-off_t mp_stream_posix_lseek(mp_obj_t stream, off_t offset, int whence) {
-    const mp_obj_base_t* o = (mp_obj_base_t*)MP_OBJ_TO_PTR(stream);
+off_t mp_stream_posix_lseek(void *stream, off_t offset, int whence) {
+    const mp_obj_base_t* o = stream;
     const mp_stream_p_t *stream_p = o->type->protocol;
     struct mp_stream_seek_t seek_s;
     seek_s.offset = offset;
     seek_s.whence = whence;
-    mp_uint_t res = stream_p->ioctl(stream, MP_STREAM_SEEK, (mp_uint_t)(uintptr_t)&seek_s, &mp_stream_errno);
+    mp_uint_t res = stream_p->ioctl(MP_OBJ_FROM_PTR(stream), MP_STREAM_SEEK, (mp_uint_t)(uintptr_t)&seek_s, &mp_stream_errno);
     if (res == MP_STREAM_ERROR) {
         return -1;
     }
     return seek_s.offset;
 }
 
-int mp_stream_posix_fsync(mp_obj_t stream) {
-    mp_obj_base_t* o = (mp_obj_base_t*)MP_OBJ_TO_PTR(stream);
+int mp_stream_posix_fsync(void *stream) {
+    mp_obj_base_t* o = stream;
     const mp_stream_p_t *stream_p = o->type->protocol;
-    mp_uint_t res = stream_p->ioctl(stream, MP_STREAM_FLUSH, 0, &mp_stream_errno);
+    mp_uint_t res = stream_p->ioctl(MP_OBJ_FROM_PTR(stream), MP_STREAM_FLUSH, 0, &mp_stream_errno);
     if (res == MP_STREAM_ERROR) {
         return -1;
     }
diff --git a/py/stream.h b/py/stream.h
index be34176db4..f4c6d30bdc 100644
--- a/py/stream.h
+++ b/py/stream.h
@@ -116,10 +116,11 @@ void mp_stream_write_adaptor(void *self, const char *buf, size_t len);
 
 #if MICROPY_STREAMS_POSIX_API
 // Functions with POSIX-compatible signatures
-ssize_t mp_stream_posix_write(mp_obj_t stream, const void *buf, size_t len);
-ssize_t mp_stream_posix_read(mp_obj_t stream, void *buf, size_t len);
-off_t mp_stream_posix_lseek(mp_obj_t stream, off_t offset, int whence);
-int mp_stream_posix_fsync(mp_obj_t stream);
+// "stream" is assumed to be a pointer to a concrete object with the stream protocol
+ssize_t mp_stream_posix_write(void *stream, const void *buf, size_t len);
+ssize_t mp_stream_posix_read(void *stream, void *buf, size_t len);
+off_t mp_stream_posix_lseek(void *stream, off_t offset, int whence);
+int mp_stream_posix_fsync(void *stream);
 #endif
 
 #if MICROPY_STREAMS_NON_BLOCK