diff --git a/geckopy/experimental/molecular_weights.py b/geckopy/experimental/molecular_weights.py
index b93f6f6..8988e3b 100644
--- a/geckopy/experimental/molecular_weights.py
+++ b/geckopy/experimental/molecular_weights.py
@@ -17,6 +17,7 @@
 import gzip
 import io
 import re
+from collections import defaultdict
 from time import sleep
 from typing import Callable, Dict, List, Optional
 
@@ -87,6 +88,9 @@ def _molecular_weight(seq: str) -> float:
     """
     seq = "".join(str(seq).split()).upper()  # Do the minimum formatting
     weight_table = protein_weights
+    weight_table = defaultdict(
+        lambda: sum(weight_table.values()) / len(weight_table), weight_table
+    )
     water = 18.010565
 
     try:
diff --git a/setup.cfg b/setup.cfg
index 88bae03..d46826f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -12,7 +12,7 @@ version = 0.0.1
 [options]
 zip_safe = True
 install_requires =
-    cobra~=0.25.0
+    cobra~=0.27.0
     pandas~=1.1.5
     tqdm~=4.60
     requests>=2