diff --git a/extmod/moduzlib.c b/extmod/moduzlib.c
index 940b72805d..f8452c72b9 100644
--- a/extmod/moduzlib.c
+++ b/extmod/moduzlib.c
@@ -48,7 +48,7 @@ typedef struct _mp_obj_decompio_t {
     bool eof;
 } mp_obj_decompio_t;
 
-STATIC unsigned char read_src_stream(TINF_DATA *data) {
+STATIC int read_src_stream(TINF_DATA *data) {
     byte *p = (void*)data;
     p -= offsetof(mp_obj_decompio_t, decomp);
     mp_obj_decompio_t *self = (mp_obj_decompio_t*)p;
@@ -110,7 +110,7 @@ STATIC mp_uint_t decompio_read(mp_obj_t o_in, void *buf, mp_uint_t size, int *er
     }
 
     o->decomp.dest = buf;
-    o->decomp.destSize = size;
+    o->decomp.dest_limit = (byte*)buf + size;
     int st = uzlib_uncompress_chksum(&o->decomp);
     if (st == TINF_DONE) {
         o->eof = true;
@@ -155,9 +155,10 @@ STATIC mp_obj_t mod_uzlib_decompress(size_t n_args, const mp_obj_t *args) {
     byte *dest_buf = m_new(byte, dest_buf_size);
 
     decomp->dest = dest_buf;
-    decomp->destSize = dest_buf_size;
-    DEBUG_printf("uzlib: Initial out buffer: " UINT_FMT " bytes\n", decomp->destSize);
+    decomp->dest_limit = dest_buf + dest_buf_size;
+    DEBUG_printf("uzlib: Initial out buffer: " UINT_FMT " bytes\n", dest_buf_size);
     decomp->source = bufinfo.buf;
+    decomp->source_limit = (byte*)bufinfo.buf + bufinfo.len;
 
     int st;
     bool is_zlib = true;
@@ -185,7 +186,7 @@ STATIC mp_obj_t mod_uzlib_decompress(size_t n_args, const mp_obj_t *args) {
         dest_buf = m_renew(byte, dest_buf, dest_buf_size, dest_buf_size + 256);
         dest_buf_size += 256;
         decomp->dest = dest_buf + offset;
-        decomp->destSize = 256;
+        decomp->dest_limit = decomp->dest + 256;
     }
 
     mp_uint_t final_sz = decomp->dest - dest_buf;