diff --git a/environs.py b/environs.py
index 4b60270..bc9d22b 100644
--- a/environs.py
+++ b/environs.py
@@ -12,6 +12,12 @@
     # Python 2
     import urlparse
 
+try:
+    from collections.abc import Mapping
+except ImportError:
+    # Python 2
+    from collections import Mapping
+
 import marshmallow as ma
 from dotenv import load_dotenv
 from dotenv.main import _walk_to_root
@@ -108,6 +114,9 @@ def _preprocess_list(value, **kwargs):
 
 
 def _preprocess_dict(value, **kwargs):
+    if isinstance(value, Mapping):
+        return value
+
     subcast = kwargs.get("subcast")
     return {
         key.strip(): subcast(val.strip()) if subcast else val.strip()
diff --git a/tests/test_environs.py b/tests/test_environs.py
index 3d54638..5c5afe0 100644
--- a/tests/test_environs.py
+++ b/tests/test_environs.py
@@ -74,6 +74,12 @@ def test_list_cast(self, set_env, env):
         set_env({"LIST": "1,2,3"})
         assert env.list("LIST") == ["1", "2", "3"]
 
+    def test_list_with_default_from_string(self, set_env, env):
+        assert env.list("LIST", "1,2") == ["1", "2"]
+
+    def test_list_with_default_from_list(self, set_env, env):
+        assert env.list("LIST", ["1"]) == ["1"]
+
     def test_list_with_subcast(self, set_env, env):
         set_env({"LIST": "1,2,3"})
         assert env.list("LIST", subcast=int) == [1, 2, 3]
@@ -100,6 +106,12 @@ def test_dict_with_subcast(self, set_env, env):
         set_env({"DICT": "key1=1,key2=2"})
         assert env.dict("DICT", subcast=int) == {"key1": 1, "key2": 2}
 
+    def test_dict_with_default_from_string(self, set_env, env):
+        assert env.dict("DICT", "key1=1,key2=2") == {"key1": "1", "key2": "2"}
+
+    def test_dict_with_default_from_dict(self, set_env, env):
+        assert env.dict("DICT", {"key1": "1"}) == {"key1": "1"}
+
     def test_decimat_cast(self, set_env, env):
         set_env({"DECIMAL": "12.34"})
         assert env.decimal("DECIMAL") == Decimal("12.34")