diff --git a/mip/mip.c b/mip/mip.c
index 16e2356a..150847d0 100644
--- a/mip/mip.c
+++ b/mip/mip.c
@@ -184,7 +184,7 @@ static void q_copyout(struct queue *q, uint8_t *buf, size_t len, size_t tail) {
 
 static bool q_write(struct queue *q, const void *buf, size_t len) {
   bool success = false;
-  size_t left = q->len - q->head + q->tail;
+  size_t left = (q->len - q->head + q->tail -1) % q->len;
   if (len + sizeof(size_t) <= left) {
     q_copyin(q, (uint8_t *) &len, sizeof(len), q->head);
     q_copyin(q, (uint8_t *) buf, len, (q->head + sizeof(size_t)) % q->len);
diff --git a/mongoose.c b/mongoose.c
index 9a307cfe..85951302 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -6472,7 +6472,7 @@ static void q_copyout(struct queue *q, uint8_t *buf, size_t len, size_t tail) {
 
 static bool q_write(struct queue *q, const void *buf, size_t len) {
   bool success = false;
-  size_t left = q->len - q->head + q->tail;
+  size_t left = (q->len - q->head + q->tail -1) % q->len;
   if (len + sizeof(size_t) <= left) {
     q_copyin(q, (uint8_t *) &len, sizeof(len), q->head);
     q_copyin(q, (uint8_t *) buf, len, (q->head + sizeof(size_t)) % q->len);