diff --git a/libc/include/stdio.h b/libc/include/stdio.h
index c12ddb8..453cf0b 100644
--- a/libc/include/stdio.h
+++ b/libc/include/stdio.h
@@ -547,7 +547,7 @@
     }
 
     // Compiler doesn't know destination size. Don't call __fgets_chk
-    if (bos == (size_t) -1) {
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
         return __fgets_real(dest, size, stream);
     }