diff --git a/basin_setup/generate_topo/main.py b/basin_setup/generate_topo/main.py
index a134c5d..58901f0 100644
--- a/basin_setup/generate_topo/main.py
+++ b/basin_setup/generate_topo/main.py
@@ -92,7 +92,7 @@ def load_basin_shapefiles(self):
         # Add the sub basin files
         if self.config['sub_basin_files'] is not None:
             for sub_basin_file in self.config['sub_basin_files']:
-                self.basin_shapefiles += Shapefile(sub_basin_file)
+                self.basin_shapefiles.append(Shapefile(sub_basin_file))
 
     def load_dem(self):
         """Reproject and crop the DEM file to a new image
diff --git a/requirements.txt b/requirements.txt
index 786f5d9..9f59084 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -12,5 +12,5 @@ netcdf4>=1.4.1
 cftime<1.1.0
 xarray>=0.16.2
 rioxarray>=0.3.1
-Shapely>=1.6.4
+Shapely>=1.6.4,<1.8
 setuptools_scm<4.2
\ No newline at end of file