diff --git a/base64.c b/base64.c
index 38ea0ab..5ed39c6 100644
--- a/base64.c
+++ b/base64.c
@@ -20,6 +20,10 @@
 	const UINT8 *src_end;
 	const UINT8 *in_pos;
 
+	if (len <= 0) {
+		return NULL;
+	}
+
 	if (!out_len) {
 		return NULL;
 	}