diff --git a/pyrobuf/parse_proto.py b/pyrobuf/parse_proto.py
index 83bef25..e3b4201 100644
--- a/pyrobuf/parse_proto.py
+++ b/pyrobuf/parse_proto.py
@@ -604,7 +604,7 @@ def add_cython_info(self, message):
                 field.setter = self.setter_map[field.type]
 
         # Determine number of bitmap fields needed to record which fields have been set
-        message.num_field_bitmaps = len(message.fields) // 64 + (1 if len(message.fields) % 64 > 0 else 0)
+        message.field_bitmap_indices = set(f.bitmap_idx for f in message.fields.values())
 
         for submessage in message.messages.values():
             self.add_cython_info(submessage)
diff --git a/pyrobuf/protobuf/templates/proto_pxd.tmpl b/pyrobuf/protobuf/templates/proto_pxd.tmpl
index 4712856..79e062e 100644
--- a/pyrobuf/protobuf/templates/proto_pxd.tmpl
+++ b/pyrobuf/protobuf/templates/proto_pxd.tmpl
@@ -33,9 +33,11 @@ cdef class {{ message.full_name }}:
     cdef {{ field.c_type }} _{{ field.name }}
     {%- endif %}
     cpdef _{{ field.name }}__reset(self)
+
 {%- endfor %}
 
-{% for i in range(message.num_field_bitmaps) %}
+
+{% for i in message.field_bitmap_indices %}
     cdef uint64_t __field_bitmap{{ i }}
 {%- endfor %}