diff --git a/src/dotenv/variables.py b/src/dotenv/variables.py
index 667f2f26..4e2ff717 100644
--- a/src/dotenv/variables.py
+++ b/src/dotenv/variables.py
@@ -62,8 +62,10 @@ def __hash__(self) -> int:
         return hash((self.__class__, self.name, self.default))
 
     def resolve(self, env: Mapping[str, Optional[str]]) -> str:
-        default = self.default if self.default is not None else ""
-        result = env.get(self.name, default)
+        result = env.get(self.name)
+        if not result and self.default is not None:
+            # POSIX ${name:-default} substitutes when name is unset OR null.
+            return self.default
         return result if result is not None else ""
 
 
diff --git a/tests/test_main.py b/tests/test_main.py
index 48dd7bf4..296d2f20 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -702,6 +702,11 @@ def test_dotenv_values_file(dotenv_path):
         # Undefined
         ({}, "a=${b}", True, {"a": ""}),
         ({}, "a=${b:-d}", True, {"a": "d"}),
+        # Defined but empty: POSIX ${name:-default} substitutes on null too
+        ({"b": ""}, "a=${b:-d}", True, {"a": "d"}),
+        ({}, "b=\na=${b:-d}", True, {"a": "d", "b": ""}),
+        ({"b": ""}, "a=${b}", True, {"a": ""}),
+        ({"b": ""}, "a=${b:-}", True, {"a": ""}),
         # With quotes
         ({"b": "c"}, 'a="${b}"', True, {"a": "c"}),
         ({"b": "c"}, "a='${b}'", True, {"a": "c"}),