diff --git a/amaranth/build/run.py b/amaranth/build/run.py
index c4b5783..25a34b9 100644
--- a/amaranth/build/run.py
+++ b/amaranth/build/run.py
@@ -33,8 +33,8 @@ class BuildPlan:
         forward slashes (``/``).
         """
         assert isinstance(filename, str) and filename not in self.files
-        if (pathlib.PurePosixPath(filename).is_absolute or
-                pathlib.PureWindowsPath(filename).is_absolute):
+        if (pathlib.PurePosixPath(filename).is_absolute() or
+                pathlib.PureWindowsPath(filename).is_absolute()):
             raise ValueError(f"Filename {filename!r} must not be an absolute path")
         self.files[filename] = content
 
@@ -83,7 +83,7 @@ class BuildPlan:
                 filename = pathlib.Path(filename)
                 # Forbid parent directory components and absolute paths completely to avoid
                 # the possibility of writing outside the build root.
-                assert not filename.is_absolute and ".." not in filename.parts
+                assert not filename.is_absolute() and ".." not in filename.parts
                 dirname = os.path.dirname(filename)
                 if dirname:
                     os.makedirs(dirname, exist_ok=True)