diff --git a/lib/uzlib/tinflate.c b/lib/uzlib/tinflate.c
index 312485fd04..53536272f3 100644
--- a/lib/uzlib/tinflate.c
+++ b/lib/uzlib/tinflate.c
@@ -185,7 +185,7 @@ unsigned char uzlib_get_byte(uzlib_uncomp_t *d)
        read next byte using it. (Note: the callback can also update ->source
        and ->source_limit). */
     if (d->source_read_cb && !d->eof) {
-        int val = d->source_read_cb(d);
+        int val = d->source_read_cb(d->source_read_data);
         if (val >= 0) {
             return (unsigned char)val;
         }
diff --git a/lib/uzlib/uzlib.h b/lib/uzlib/uzlib.h
index 030f7698c1..16984a77d3 100644
--- a/lib/uzlib/uzlib.h
+++ b/lib/uzlib/uzlib.h
@@ -84,7 +84,8 @@ typedef struct _uzlib_uncomp_t {
        also return -1 in case of EOF (or irrecoverable error). Note that
        besides returning the next byte, it may also update source and
        source_limit fields, thus allowing for buffered operation. */
-    int (*source_read_cb)(struct _uzlib_uncomp_t *uncomp);
+    void *source_read_data;
+    int (*source_read_cb)(void *);
 
     unsigned int tag;
     unsigned int bitcount;
diff --git a/ports/stm32/mboot/gzstream.c b/ports/stm32/mboot/gzstream.c
index 764ef0a477..f8b0987edf 100644
--- a/ports/stm32/mboot/gzstream.c
+++ b/ports/stm32/mboot/gzstream.c
@@ -45,7 +45,8 @@ typedef struct _gz_stream_t {
 
 static gz_stream_t gz_stream SECTION_NOZERO_BSS;
 
-static int gz_stream_read_src(uzlib_uncomp_t *decomp) {
+static int gz_stream_read_src(void *data) {
+    uzlib_uncomp_t *decomp = data;
     int n = gz_stream.stream_read(gz_stream.stream_data, gz_stream.buf, sizeof(gz_stream.buf));
     if (n < 0) {
         // Stream error
@@ -76,6 +77,7 @@ int gz_stream_init_from_stream(void *stream_data, stream_read_t stream_read) {
     gz_stream.stream_read = stream_read;
 
     memset(&gz_stream.decomp, 0, sizeof(gz_stream.decomp));
+    gz_stream.decomp.source_read_data = &gz_stream.decomp;
     gz_stream.decomp.source_read_cb = gz_stream_read_src;
 
     int header_wbits;