diff --git a/src/kernel/libroot/os/debug.c b/src/kernel/libroot/os/debug.c
index 821cbb2a35..745b197d15 100644
--- a/src/kernel/libroot/os/debug.c
+++ b/src/kernel/libroot/os/debug.c
@@ -107,7 +107,7 @@ _sPrintf(const char *fmt, ...)
 	ret = vsnprintf(buffer, 256, fmt, ap);
 	va_end(ap);
 
-	if (!ret)
+	if (ret < 0)
 		return ret;
 
 	_kern_debug_output(buffer);