diff --git a/library/debug.c b/library/debug.c
index c7bbd41..a9d58e5 100644
--- a/library/debug.c
+++ b/library/debug.c
@@ -366,7 +366,7 @@
     start = text;
     for (cur = text; *cur != '\0'; cur++) {
         if (*cur == '\n') {
-            size_t len = cur - start + 1;
+            size_t len = (size_t) (cur - start) + 1;
             if (len > DEBUG_BUF_SIZE - 1) {
                 len = DEBUG_BUF_SIZE - 1;
             }