diff --git a/mip/mip.c b/mip/mip.c
index 1b591076..7e340fe7 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 916f0925..81738650 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);
diff --git a/test/mip_test.c b/test/mip_test.c
index 2de65eb3..3bb05940 100644
--- a/test/mip_test.c
+++ b/test/mip_test.c
@@ -2,15 +2,17 @@
 #include "mongoose.c"
 
 static void test_queue(void) {
-  uint8_t buf[sizeof(size_t) + 5];
+  static uint8_t buf[sizeof(size_t) + sizeof(uint16_t) + 3 ]; // fit 1 element but not 2
   uint16_t val = 1234;
-  struct queue q = {buf, sizeof(buf), 0, 0};
+  static struct queue q = {buf, sizeof(buf), 0, 0};
 
   // Write to an empty queue, and read back
   assert(q_avail(&q) == 0);
   assert(q_write(&q, &val, sizeof(val)) == true);
   assert(q_avail(&q) == sizeof(val));
   assert(q.head > q.tail);
+  // Only one element may fit
+  assert(q_write(&q, &val, sizeof(val)) == false);
   val = 0;
   assert(q_read(&q, &val) == sizeof(val));
   assert(val == 1234);
@@ -20,6 +22,8 @@ static void test_queue(void) {
   assert(q_write(&q, &val, sizeof(val)) == true);
   assert(q_avail(&q) == sizeof(val));
   assert(q.head < q.tail);
+  // Only one element may fit
+  assert(q_write(&q, &val, sizeof(val)) == false);
   val = 0;
   assert(q_read(&q, &val) == sizeof(val));
   assert(val == 1234);