diff --git a/src/argh/assembling.py b/src/argh/assembling.py
index db17a6a..93c7f19 100644
--- a/src/argh/assembling.py
+++ b/src/argh/assembling.py
@@ -415,7 +415,10 @@ def _merge_inferred_and_declared_args(
                 kind_declared = kinds[decl_positional]
                 raise AssemblingError(
                     f'argument "{func_arg_name}" declared as {kind_inferred} '
-                    f"(in function signature) and {kind_declared} (via decorator)"
+                    f"(in function signature) and {kind_declared} (via decorator). "
+                    "If you've just migrated from Argh v.0.29, please check "
+                    "the new default NameMappingPolicy. Perhaps you need "
+                    "to replace `func(x=1)` with `func(*, x=1)`?"
                 )
 
             # merge explicit argument declaration into the inferred one
diff --git a/tests/test_regressions.py b/tests/test_regressions.py
index eaa3a6d..6e4314b 100644
--- a/tests/test_regressions.py
+++ b/tests/test_regressions.py
@@ -116,7 +116,9 @@ def func(foo_bar):
         parser.set_default_command(func)
     msg = (
         'func: argument "foo_bar" declared as positional (in function '
-        "signature) and optional (via decorator)"
+        "signature) and optional (via decorator). If you've just migrated "
+        "from Argh v.0.29, please check the new default NameMappingPolicy. "
+        "Perhaps you need to replace `func(x=1)` with `func(*, x=1)`?"
     )
     assert excinfo.exconly().endswith(msg)