diff --git a/amulet_nbt/_util.pyx b/amulet_nbt/_util.pyx
index 7d171ba3..b8d87034 100644
--- a/amulet_nbt/_util.pyx
+++ b/amulet_nbt/_util.pyx
@@ -87,16 +87,21 @@ cdef inline void write_string(str s, object buffer, bint little_endian, string_e
 
 cdef inline void write_bytes(bytes b, object buffer, bint little_endian):
     cdef char *c = b
-    cdef short length = <short> len(b)
-    write_short(length, buffer, little_endian)
+    cdef size_t length = len(b)
+    if length > 2**16-1:
+        raise RuntimeError("String cannot be longer than 2**16 - 1")
+    write_short(<short> length, buffer, little_endian)
     cwrite(buffer, c, length)
 
 cdef inline void write_array(object value, object buffer, char size, bint little_endian):
     value = value.tobytes()
     cdef char*s = value
-    cdef int length = <int> len(value) // size
-    to_little_endian(&length, 4, little_endian)
-    cwrite(buffer, <char*> &length, 4)
+    cdef size_t length = len(value) // size
+    if length > 2**31-1:
+        raise RuntimeError("Array cannot be longer than 2**31 - 1")
+    cdef int length_int = <int> length
+    to_little_endian(&length_int, 4, little_endian)
+    cwrite(buffer, <char*> &length_int, 4)
     cwrite(buffer, s, len(value))
 
 cdef inline void write_byte(char value, object buffer):
diff --git a/tests/tags/test_string.py b/tests/tags/test_string.py
index 221baaf1..0757ce1d 100644
--- a/tests/tags/test_string.py
+++ b/tests/tags/test_string.py
@@ -188,6 +188,15 @@ def test_to_nbt(self):
             ),
         )
 
+        # Test writing long strings
+        self.assertEqual(
+            b"\x08\x00\x00\xff\xff" + b"a"*65535,
+            StringTag("a"*65535).to_nbt(compressed=False)
+        )
+
+        with self.assertRaises(RuntimeError):
+            StringTag("a" * 65536).to_nbt(compressed=False)
+
     def test_from_nbt(self):
         self.assertStrictEqual(
             StringTag(),