diff --git a/library/entropy.c b/library/entropy.c
index 7e25f28..e55410c 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -677,7 +677,7 @@
         goto cleanup;
     }
 
-    if ((ret = mbedtls_entropy_update_manual(&ctx, buf, sizeof buf)) != 0) {
+    if ((ret = mbedtls_entropy_update_manual(&ctx, buf, sizeof(buf))) != 0) {
         goto cleanup;
     }