diff --git a/mibuild/altera_quartus.py b/mibuild/altera_quartus.py
index 4fc785509..dbb336a75 100644
--- a/mibuild/altera_quartus.py
+++ b/mibuild/altera_quartus.py
@@ -74,6 +74,7 @@ quartus_sta {build_name}.qpf
 class AlteraQuartusPlatform(GenericPlatform):
 	def build(self, fragment, build_dir="build", build_name="top",
 			quartus_path="/opt/Altera", run=True):
+		self.finalize(fragment)
 		tools.mkdir_noerror(build_dir)
 		os.chdir(build_dir)
 
diff --git a/mibuild/generic_platform.py b/mibuild/generic_platform.py
index 482c485e8..2feb70e4d 100644
--- a/mibuild/generic_platform.py
+++ b/mibuild/generic_platform.py
@@ -152,6 +152,7 @@ class GenericPlatform:
 			name = self.__module__.split(".")[-1]
 		self.name = name
 		self.sources = []
+		self.finalized = False
 
 	def request(self, *args, **kwargs):
 		return self.constraint_manager.request(*args, **kwargs)
@@ -162,6 +163,17 @@ class GenericPlatform:
 	def add_platform_command(self, *args, **kwargs):
 		return self.constraint_manager.add_platform_command(*args, **kwargs)
 
+	def finalize(self, fragment, *args, **kwargs):
+		if self.finalized:
+			raise ConstraintError("Already finalized")
+		self.do_finalize(fragment, *args, **kwargs)
+		self.finalized = True
+
+	def do_finalize(self, fragment, *args, **kwargs):
+		"""overload this and e.g. add_platform_command()'s after the
+		modules had their say"""
+		pass
+
 	def add_source(self, filename, language=None):
 		if language is None:
 			language = tools.language_by_filename(filename)
diff --git a/mibuild/xilinx_ise.py b/mibuild/xilinx_ise.py
index bb70fdd3c..2bc8c107f 100644
--- a/mibuild/xilinx_ise.py
+++ b/mibuild/xilinx_ise.py
@@ -150,6 +150,7 @@ class XilinxISEPlatform(GenericPlatform):
 
 	def build(self, fragment, build_dir="build", build_name="top",
 			ise_path="/opt/Xilinx", source=True, run=True):
+		self.finalize(fragment)
 		tools.mkdir_noerror(build_dir)
 		os.chdir(build_dir)