diff --git a/lib/misc/base64-decode.c b/lib/misc/base64-decode.c
index 9d18b33fc..332e91dfb 100644
--- a/lib/misc/base64-decode.c
+++ b/lib/misc/base64-decode.c
@@ -113,7 +113,7 @@ lws_b64_decode_stateful(struct lws_b64state *s, const char *in, size_t *in_len,
 	const char *orig_in = in, *end_in = in + *in_len;
 	uint8_t *orig_out = out, *end_out = out + *out_size;
 
-	while (in < end_in && *in && out + 4 < end_out) {
+	while (in < end_in && *in && out + 3 <= end_out) {
 
 		for (; s->i < 4 && in < end_in && *in; s->i++) {
 			uint8_t v;