diff --git a/nmigen/build/plat.py b/nmigen/build/plat.py
index 174cda7..422cc85 100644
--- a/nmigen/build/plat.py
+++ b/nmigen/build/plat.py
@@ -8,6 +8,7 @@ import jinja2
 from .. import __version__
 from .._toolchain import *
 from ..hdl import *
+from ..hdl.xfrm import SampleLowerer, DomainLowerer
 from ..lib.cdc import ResetSynchronizer
 from ..back import rtlil, verilog
 from .res import *
@@ -115,7 +116,9 @@ class Platform(ResourceManager, metaclass=ABCMeta):
         self._prepared = True
 
         fragment = Fragment.get(elaboratable, self)
-        fragment.create_missing_domains(self.create_missing_domain, platform=self)
+        fragment = SampleLowerer()(fragment)
+        fragment._propagate_domains(self.create_missing_domain, platform=self)
+        fragment = DomainLowerer()(fragment)
 
         def add_pin_fragment(pin, pin_fragment):
             pin_fragment = Fragment.get(pin_fragment, self)
@@ -144,7 +147,7 @@ class Platform(ResourceManager, metaclass=ABCMeta):
                 add_pin_fragment(pin,
                     self.get_diff_input_output(pin, p_port, n_port, attrs, invert))
 
-        fragment = fragment.prepare(ports=self.iter_ports(), missing_domain=lambda name: None)
+        fragment._propagate_ports(ports=self.iter_ports(), all_undef_as_ports=False)
         return self.toolchain_prepare(fragment, name, **kwargs)
 
     @abstractmethod
diff --git a/nmigen/hdl/ir.py b/nmigen/hdl/ir.py
index cea9b40..8c169ad 100644
--- a/nmigen/hdl/ir.py
+++ b/nmigen/hdl/ir.py
@@ -343,7 +343,7 @@ class Fragment:
 
             subfrag._propagate_domains_down()
 
-    def create_missing_domains(self, missing_domain, *, platform=None):
+    def _create_missing_domains(self, missing_domain, *, platform=None):
         from .xfrm import DomainCollector
 
         collector = DomainCollector()
@@ -373,11 +373,11 @@ class Fragment:
                 self.add_domains(new_fragment.domains.values())
         return new_domains
 
-    def _propagate_domains(self, missing_domain):
+    def _propagate_domains(self, missing_domain, *, platform=None):
         self._propagate_domains_up()
         self._propagate_domains_down()
         self._resolve_hierarchy_conflicts()
-        new_domains = self.create_missing_domains(missing_domain)
+        new_domains = self._create_missing_domains(missing_domain, platform=platform)
         self._propagate_domains_down()
         return new_domains