diff --git a/borghash.pxd b/borghash.pxd
index a71b212..1a3584d 100644
--- a/borghash.pxd
+++ b/borghash.pxd
@@ -20,7 +20,7 @@ cdef class HashTable:
 
 cdef class HashTableNT:
     cdef int key_size
-    cdef str value_format
     cdef object value_type
+    cdef object value_struct
     cdef int value_size
     cdef HashTable inner
diff --git a/borghash.pyx b/borghash.pyx
index 291e160..c03e5ce 100644
--- a/borghash.pyx
+++ b/borghash.pyx
@@ -348,8 +348,8 @@ cdef class HashTableNT:
         if value_type is None:
             raise ValueError("value_type must be specified (a namedtuple type corresponding to value_format).")
         self.key_size = key_size
-        self.value_format = value_format
-        self.value_size = struct.calcsize(self.value_format)
+        self.value_struct = struct.Struct(value_format)
+        self.value_size = self.value_struct.size
         self.value_type = value_type
         self.inner = HashTable(key_size=self.key_size, value_size=self.value_size, capacity=capacity)
         _fill(self, items)
@@ -369,10 +369,10 @@ cdef class HashTableNT:
                 value = self.value_type(*value)
             else:
                 raise TypeError(f"Expected an instance of {self.value_type}, got {type(value)}")
-        return struct.pack(self.value_format, *value)
+        return self.value_struct.pack(*value)
 
     def _to_namedtuple_value(self, binary_value: bytes) -> Any:
-        unpacked_data = struct.unpack(self.value_format, binary_value)
+        unpacked_data = self.value_struct.unpack(binary_value)
         return self.value_type(*unpacked_data)
 
     def _set_raw(self, key: bytes, value: bytes) -> None:
@@ -460,7 +460,7 @@ cdef class HashTableNT:
         meta = {
             'key_size': self.key_size,
             'value_size': self.value_size,
-            'value_format': self.value_format,
+            'value_format': self.value_struct.format,
             'value_type_name': self.value_type.__name__,
             'value_type_fields': self.value_type._fields,
             'capacity': self.inner.capacity,