diff --git a/library/ecdh.c b/library/ecdh.c
index e060b18..52b1617 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -363,7 +363,7 @@
                                      const unsigned char *end)
 {
     return mbedtls_ecp_tls_read_point(&ctx->grp, &ctx->Qp, buf,
-                                      end - *buf);
+                                      (size_t) (end - *buf));
 }
 
 /*
@@ -379,7 +379,7 @@
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     mbedtls_ecp_group_id grp_id;
-    if ((ret = mbedtls_ecp_tls_read_group_id(&grp_id, buf, end - *buf))
+    if ((ret = mbedtls_ecp_tls_read_group_id(&grp_id, buf, (size_t) (end - *buf)))
         != 0) {
         return ret;
     }