diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index e439c5b..0189e77 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -951,10 +951,10 @@
         Py_BEGIN_ALLOW_THREADS
         /* Avoid truncation of length for very large buffers. adler32() takes
            length as an unsigned int, which may be narrower than Py_ssize_t. */
-        while (len > (Py_ssize_t)UINT_MAX) {
+        while (len > (size_t) UINT_MAX) {
             adler32val = adler32(adler32val, buf, UINT_MAX);
-            buf += UINT_MAX;
-            len -= UINT_MAX;
+            buf += (size_t) UINT_MAX;
+            len -= (size_t) UINT_MAX;
         }
         adler32val = adler32(adler32val, buf, len);
         Py_END_ALLOW_THREADS
@@ -989,10 +989,10 @@
         Py_BEGIN_ALLOW_THREADS
         /* Avoid truncation of length for very large buffers. crc32() takes
            length as an unsigned int, which may be narrower than Py_ssize_t. */
-        while (len > (Py_ssize_t)UINT_MAX) {
+        while (len > (size_t) UINT_MAX) {
             crc32val = crc32(crc32val, buf, UINT_MAX);
-            buf += UINT_MAX;
-            len -= UINT_MAX;
+            buf += (size_t) UINT_MAX;
+            len -= (size_t) UINT_MAX;
         }
         signed_val = crc32(crc32val, buf, len);
         Py_END_ALLOW_THREADS