Index: kernel/generic/src/printf/printf_core.c
===================================================================
--- kernel/generic/src/printf/printf_core.c	(revision 1b0b48e09eab2e821aca4d6f6b078ce38136134f)
+++ kernel/generic/src/printf/printf_core.c	(revision 06b785f8cd841eaeec20587b753477850c12723b)
@@ -126,7 +126,7 @@
 {
 	if (str == NULL)
-		return printf_putnchars_utf8(nullstr, strlen(nullstr), ps);
-	
-	return ps->write_utf8((void *) str, strlen(str), ps->data);
+		return printf_putnchars_utf8(nullstr, str_size(nullstr), ps);
+	
+	return ps->write_utf8((void *) str, str_size(str), ps->data);
 }