diff --git a/libc/src/unistd.cpp b/libc/src/unistd.cpp
index 75e8cedb..5c80ae87 100644
--- a/libc/src/unistd.cpp
+++ b/libc/src/unistd.cpp
@@ -473,7 +473,7 @@ extern "C"
         long rc = syscall(SYS_isatty, fd);
         if (rc < 0)
         {
-            errno = -rc;
+            errno = (int)-rc;
             return 0;
         }
         return rc;