diff --git a/src/rez/rex.py b/src/rez/rex.py
index da26ef5185..24cdc7cbb2 100644
--- a/src/rez/rex.py
+++ b/src/rez/rex.py
@@ -1114,8 +1114,15 @@ def __init__(self, manager) -> None:
         self._var_cache = dict((k, EnvironmentVariable(k, self))
                                for k in manager.parent_environ.keys())
 
+    def _defined_keys(self):
+        # A key is cached on first access, even if it was never defined (eg a
+        # package's commands() referencing a variable that does not exist).
+        # Such keys must not be exposed by the mapping interface, otherwise
+        # iterating the environment raises RexUndefinedVariableError.
+        return [k for k in self._var_cache if not self.manager.undefined(k)]
+
     def keys(self):
-        return self._var_cache.keys()
+        return dict.fromkeys(self._defined_keys()).keys()
 
     def __repr__(self) -> str:
         return '%s(%s)' % (self.__class__.__name__, str(self._var_cache))
@@ -1129,17 +1136,17 @@ def __setitem__(self, key, value) -> None:
         self[key].set(value)
 
     def __contains__(self, key) -> bool:
-        return (key in self._var_cache)
+        return (key in self._var_cache and not self.manager.undefined(key))
 
     def __delitem__(self, key) -> None:
         del self._var_cache[key]
 
     def __iter__(self):
-        for key in self._var_cache.keys():
+        for key in self._defined_keys():
             yield key
 
     def __len__(self) -> int:
-        return len(self._var_cache)
+        return len(self._defined_keys())
 
 
 class EnvironmentVariable(object):
diff --git a/src/rez/tests/test_rex.py b/src/rez/tests/test_rex.py
index ac3a8966df..de12d332b3 100644
--- a/src/rez/tests/test_rex.py
+++ b/src/rez/tests/test_rex.py
@@ -390,6 +390,27 @@ def _rex_3():
         _test(_rex_2, env={"A": "foo"}, expected={"A": True, "B": False})
         _test(_rex_3, env={}, expected="not b")
 
+    def test_11(self) -> None:
+        """Test that a failed env lookup doesn't leak into the env mapping."""
+
+        def _rex():
+            try:
+                env.NOT_SET
+            except Exception:
+                pass
+            return {
+                "keys": sorted(env.keys()),
+                "contains": "NOT_SET" in env,
+                "len": len(env),
+                "values": [str(x) for x in env.values()],
+            }
+
+        ex = self._create_executor(env={"A": "foo"})
+        self.assertEqual(
+            ex.execute_function(_rex),
+            {"keys": ["A"], "contains": False, "len": 1, "values": ["foo"]},
+        )
+
     def test_version_binding(self) -> None:
         """Test the Rex binding of the Version class."""
         v = VersionBinding(Version("1.2.3alpha"))