diff --git a/py/modstruct.c b/py/modstruct.c
index 0d4a45f6b6..61dd0f81b4 100644
--- a/py/modstruct.c
+++ b/py/modstruct.c
@@ -82,26 +82,10 @@ STATIC mp_uint_t get_fmt_num(const char **p) {
     return val;
 }
 
-STATIC uint calcsize_items(const char *fmt) {
-    uint cnt = 0;
-    while (*fmt) {
-        int num = 1;
-        if (unichar_isdigit(*fmt)) {
-            num = get_fmt_num(&fmt);
-            if (*fmt == 's') {
-                num = 1;
-            }
-        }
-        cnt += num;
-        fmt++;
-    }
-    return cnt;
-}
-
-STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
-    const char *fmt = mp_obj_str_get_str(fmt_in);
+STATIC size_t calc_size_items(const char *fmt, size_t *total_sz) {
     char fmt_type = get_fmt_type(&fmt);
-    mp_uint_t size;
+    size_t total_cnt = 0;
+    size_t size;
     for (size = 0; *fmt; fmt++) {
         mp_uint_t cnt = 1;
         if (unichar_isdigit(*fmt)) {
@@ -109,8 +93,10 @@ STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
         }
 
         if (*fmt == 's') {
+            total_cnt += 1;
             size += cnt;
         } else {
+            total_cnt += cnt;
             mp_uint_t align;
             size_t sz = mp_binary_get_size(fmt_type, *fmt, &align);
             while (cnt--) {
@@ -120,6 +106,14 @@ STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
             }
         }
     }
+    *total_sz = size;
+    return total_cnt;
+}
+
+STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
+    const char *fmt = mp_obj_str_get_str(fmt_in);
+    size_t size;
+    calc_size_items(fmt, &size);
     return MP_OBJ_NEW_SMALL_INT(size);
 }
 MP_DEFINE_CONST_FUN_OBJ_1(struct_calcsize_obj, struct_calcsize);
@@ -130,8 +124,9 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
     // Since we implement unpack and unpack_from using the same function
     // we relax the "exact" requirement, and only implement "big enough".
     const char *fmt = mp_obj_str_get_str(args[0]);
+    size_t total_sz;
+    size_t num_items = calc_size_items(fmt, &total_sz);
     char fmt_type = get_fmt_type(&fmt);
-    uint num_items = calcsize_items(fmt);
     mp_obj_tuple_t *res = MP_OBJ_TO_PTR(mp_obj_new_tuple(num_items, NULL));
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[1], &bufinfo, MP_BUFFER_READ);
@@ -152,21 +147,23 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
         p += offset;
     }
 
-    for (uint i = 0; i < num_items;) {
-        mp_uint_t sz = 1;
+    // Check that the input buffer is big enough to unpack all the values
+    if (p + total_sz > end_p) {
+        mp_raise_ValueError("buffer too small");
+    }
+
+    for (size_t i = 0; i < num_items;) {
+        mp_uint_t cnt = 1;
         if (unichar_isdigit(*fmt)) {
-            sz = get_fmt_num(&fmt);
-        }
-        if (p + sz > end_p) {
-            mp_raise_ValueError("buffer too small");
+            cnt = get_fmt_num(&fmt);
         }
         mp_obj_t item;
         if (*fmt == 's') {
-            item = mp_obj_new_bytes(p, sz);
-            p += sz;
+            item = mp_obj_new_bytes(p, cnt);
+            p += cnt;
             res->items[i++] = item;
         } else {
-            while (sz--) {
+            while (cnt--) {
                 item = mp_binary_get_val(fmt_type, *fmt, &p);
                 res->items[i++] = item;
             }
diff --git a/tests/basics/struct1.py b/tests/basics/struct1.py
index a442beb1e5..2cf75137b8 100644
--- a/tests/basics/struct1.py
+++ b/tests/basics/struct1.py
@@ -39,6 +39,12 @@ print(v == (10, 100, 200, 300))
 # network byte order
 print(struct.pack('!i', 123))
 
+# check that we get an error if the buffer is too small
+try:
+    struct.unpack('I', b'\x00\x00\x00')
+except:
+    print('struct.error')
+
 # first arg must be a string
 try:
     struct.pack(1, 2)
diff --git a/tests/basics/struct2.py b/tests/basics/struct2.py
index 3b9dd5c1f6..e3336c0c78 100644
--- a/tests/basics/struct2.py
+++ b/tests/basics/struct2.py
@@ -25,6 +25,12 @@ print(struct.calcsize('0s1s0H2H'))
 print(struct.unpack('<0s1s0H2H', b'01234'))
 print(struct.pack('<0s1s0H2H', b'abc', b'abc', 258, 515))
 
+# check that we get an error if the buffer is too small
+try:
+    struct.unpack('2H', b'\x00\x00')
+except:
+    print('Exception')
+
 # check that unknown types raise an exception
 try:
     struct.unpack('z', b'1')