diff --git a/Modules/sha1module.c b/Modules/sha1module.c
index b25bd44..1cace54 100644
--- a/Modules/sha1module.c
+++ b/Modules/sha1module.c
@@ -218,7 +218,7 @@
            in             += SHA1_BLOCKSIZE;
            inlen          -= SHA1_BLOCKSIZE;
         } else {
-           n = MIN(inlen, (SHA1_BLOCKSIZE - sha1->curlen));
+           n = MIN(inlen, (Py_ssize_t)(SHA1_BLOCKSIZE - sha1->curlen));
            memcpy(sha1->buf + sha1->curlen, in, (size_t)n);
            sha1->curlen   += n;
            in             += n;