diff --git a/common/cmd_tpm.c b/common/cmd_tpm.c
index 65e7371..e9c6618 100644
--- a/common/cmd_tpm.c
+++ b/common/cmd_tpm.c
@@ -58,6 +58,8 @@
 	size_t count, length;
 	int i;
 
+	if (!bytes)
+		return NULL;
 	length = strlen(bytes);
 	count = length / 2;