diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index ec82ec95d..d4aacbd1a 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -28,6 +28,7 @@ from .interpreterbase import InterpreterBase
 from .interpreterbase import check_stringlist, noPosargs, noKwargs, stringArgs
 from .interpreterbase import InterpreterException, InvalidArguments, InvalidCode
 from .interpreterbase import InterpreterObject, MutableInterpreterObject
+from .modules import ModuleReturnValue
 
 import os, sys, shutil, uuid
 import re
@@ -973,6 +974,7 @@ class ModuleHolder(InterpreterObject):
             raise InvalidArguments('Module %s does not have method %s.' % (self.modname, method_name))
         if method_name.startswith('_'):
             raise InvalidArguments('Function {!r} in module {!r} is private.'.format(method_name, self.modname))
+        build_hash = hash(self.interpreter.build)
         state = ModuleState()
         state.build_to_src = os.path.relpath(self.interpreter.environment.get_source_dir(),
                                              self.interpreter.environment.get_build_dir())
@@ -988,6 +990,8 @@ class ModuleHolder(InterpreterObject):
         state.global_args = self.interpreter.build.global_args
         state.project_args = self.interpreter.build.projects_args.get(self.interpreter.subproject, {})
         value = fn(state, args, kwargs)
+        if hash(self.interpreter.build) != build_hash:
+            raise InterpreterException('Extension module altered internal state illegally.')
         return self.interpreter.module_method_callback(value)
 
 class MesonMain(InterpreterObject):
@@ -1227,7 +1231,10 @@ class Interpreter(InterpreterBase):
                            'join_paths': self.func_join_paths,
                            })
 
-    def module_method_callback(self, invalues):
+    def module_method_callback(self, return_object):
+        if not isinstance(return_object, ModuleReturnValue):
+            raise InterpreterException('Bug in module, it returned an invalid object')
+        invalues = return_object.new_objects
         unwrap_single = False
         if invalues is None:
             return
@@ -1264,7 +1271,7 @@ class Interpreter(InterpreterBase):
                 raise InterpreterException('Module returned a value of unknown type.')
         if len(outvalues) == 1 and unwrap_single:
             return outvalues[0]
-        return outvalues
+        return return_object.return_value
 
     def get_build_def_files(self):
         return self.build_def_files
diff --git a/mesonbuild/modules/__init__.py b/mesonbuild/modules/__init__.py
index 184d85aef..16cada08d 100644
--- a/mesonbuild/modules/__init__.py
+++ b/mesonbuild/modules/__init__.py
@@ -45,6 +45,11 @@ def get_include_args(environment, include_dirs, prefix='-I'):
 
     return dirs_str
 
+class ModuleReturnValue:
+    def __init__(self, return_value, new_objects):
+        self.return_value = return_value
+        assert(isinstance(new_objects, list))
+        self.new_objects = new_objects
 
 class GResourceTarget(build.CustomTarget):
     def __init__(self, name, subdir, kwargs):
diff --git a/mesonbuild/modules/modtest.py b/mesonbuild/modules/modtest.py
index c9247e6bf..dc347e236 100644
--- a/mesonbuild/modules/modtest.py
+++ b/mesonbuild/modules/modtest.py
@@ -12,10 +12,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from . import ModuleReturnValue
+
 class TestModule:
 
     def print_hello(self, state, args, kwargs):
         print('Hello from a Meson module')
+        rv = ModuleReturnValue(None, [])
+        return rv
 
 def initialize():
     return TestModule()
diff --git a/mesonbuild/modules/pkgconfig.py b/mesonbuild/modules/pkgconfig.py
index aaf0746c7..38358f3b2 100644
--- a/mesonbuild/modules/pkgconfig.py
+++ b/mesonbuild/modules/pkgconfig.py
@@ -15,6 +15,8 @@
 from .. import build
 from .. import mesonlib
 from .. import mlog
+from . import ModuleReturnValue
+
 import os
 
 class PkgConfigModule:
@@ -138,7 +140,8 @@ class PkgConfigModule:
         self.generate_pkgconfig_file(state, libs, subdirs, name, description, url,
                                      version, pcfile, pub_reqs, priv_reqs,
                                      conflicts, priv_libs)
-        return build.Data(mesonlib.File(True, state.environment.get_scratch_dir(), pcfile), pkgroot)
+        res = build.Data(mesonlib.File(True, state.environment.get_scratch_dir(), pcfile), pkgroot)
+        return ModuleReturnValue(res, [res])
 
 def initialize():
     return PkgConfigModule()