diff --git a/gdplib/gdp_col/__init__.py b/gdplib/gdp_col/__init__.py
index b508512..127f4d2 100644
--- a/gdplib/gdp_col/__init__.py
+++ b/gdplib/gdp_col/__init__.py
@@ -23,6 +23,7 @@ def build_model():
 
     m.BigM = Suffix(direction=Suffix.LOCAL)
     m.BigM[None] = 100
+    return m
 
 
 __all__ = ['build_model']