More explicit failures

pipenv/core.py

@@ -102,7 +102,11 @@ if PIPENV_NOSPIN:
 
 def which(command, location=None, allow_global=False):
     if not allow_global and location is None:
-        location = project.virtualenv_location or os.environ.get("VIRTUAL_ENV")
+        location = (
+            project.virtualenv_location
+            or os.environ.get("VIRTUAL_ENV", "")
+        )
+    assert location, "virtual environment not created"
     if not allow_global:
         if os.name == "nt":
             p = find_windows_executable(os.path.join(location, "Scripts"), command)

pipenv/project.py

@@ -319,12 +319,14 @@ class Project(object):
         if self._virtualenv_location:
             return self._virtualenv_location
 
+        assert self.project_directory, "project not created"
+
         # Default mode.
         if not self.is_venv_in_project():
             loc = self._get_virtualenv_location(self.virtualenv_name)
         # The user wants the virtualenv in the project.
         else:
-            loc = os.sep.join(self.pipfile_location.split(os.sep)[:-1] + [".venv"])
+            loc = os.path.join(self.project_directory, ".venv")
         self._virtualenv_location = loc
         return loc