diff --git a/arch/sandbox/os/common.c b/arch/sandbox/os/common.c
index 8611882..1b2daa3 100644
--- a/arch/sandbox/os/common.c
+++ b/arch/sandbox/os/common.c
@@ -264,7 +264,7 @@
 			PROT_READ | (readonly ? 0 : PROT_WRITE),
 			MAP_SHARED, fd, 0);
 	if ((void *)hf->base == MAP_FAILED)
-		printf("warning: mmapping %s failed\n", filename);
+		printf("warning: mmapping %s failed: %s\n", filename, strerror(errno));
 
 	ret = barebox_register_filedev(hf);
 	if (ret)