Include prefix in the ImproperlyConfigured error

Author: spenserblack

environ/environ.py

@@ -388,7 +388,7 @@ def get_value(self, var, cast=None, default=NOTSET, parse_default=False):
             value = self.ENVIRON[var_name]
         except KeyError as exc:
             if default is self.NOTSET:
-                error_msg = f'Set the {var} environment variable'
+                error_msg = f'Set the {var_name} environment variable'
                 raise ImproperlyConfigured(error_msg) from exc
 
             value = default

tests/test_env.py

@@ -407,6 +407,13 @@ def test_prefix(self):
         self.env.prefix = 'PREFIX_'
         assert self.env('TEST') == 'foo'
 
+    def test_prefix_and_not_present_without_default(self):
+        self.env.prefix = 'PREFIX_'
+        with pytest.raises(ImproperlyConfigured) as excinfo:
+            self.env('not_present')
+        assert str(excinfo.value) == 'Set the PREFIX_not_present environment variable'
+        assert excinfo.value.__cause__ is not None
+
 
 class TestFileEnv(TestEnv):
     def setup_method(self, method):