diff --git a/rocrate/model/dataset.py b/rocrate/model/dataset.py
index f883a02..b378ea3 100644
--- a/rocrate/model/dataset.py
+++ b/rocrate/model/dataset.py
@@ -18,6 +18,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import errno
+import os
 import shutil
 from pathlib import Path
 from urllib.request import urlopen
@@ -48,9 +50,16 @@ class Dataset(FileOrDir):
             if self.fetch_remote:
                 self.__get_parts(out_path)
         else:
-            out_path.mkdir(parents=True, exist_ok=True)
-            if not self.crate.source and self.source and Path(self.source).exists():
-                self.crate._copy_unlisted(self.source, out_path)
+            if self.source is None:
+                out_path.mkdir(parents=True, exist_ok=True)
+            else:
+                if not Path(self.source).exists():
+                    raise FileNotFoundError(
+                        errno.ENOENT, os.strerror(errno.ENOENT), str(self.source)
+                    )
+                out_path.mkdir(parents=True, exist_ok=True)
+                if not self.crate.source:
+                    self.crate._copy_unlisted(self.source, out_path)
 
     def __get_parts(self, out_path):
         out_path.mkdir(parents=True, exist_ok=True)
diff --git a/rocrate/model/file.py b/rocrate/model/file.py
index 263677d..6ea48db 100644
--- a/rocrate/model/file.py
+++ b/rocrate/model/file.py
@@ -18,10 +18,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import os
 from pathlib import Path
 import shutil
 import urllib.request
+import warnings
 from http.client import HTTPResponse
 from io import BytesIO, StringIO
 
@@ -59,7 +59,10 @@ class File(FileOrDir):
                     if self.fetch_remote:
                         out_file_path.parent.mkdir(parents=True, exist_ok=True)
                         urllib.request.urlretrieve(response.url, out_file_path)
-        elif os.path.isfile(self.source):
+        elif self.source is None:
+            # Allows to record a File entity whose @id does not exist, see #73
+            warnings.warn(f"No source for {self.id}")
+        else:
             out_file_path.parent.mkdir(parents=True, exist_ok=True)
             if not out_file_path.exists() or not out_file_path.samefile(self.source):
                 shutil.copy(self.source, out_file_path)

diff --git a/test/test_read.py b/test/test_read.py
index edb1d9f..df50801 100644
--- a/test/test_read.py
+++ b/test/test_read.py
@@ -320,10 +320,27 @@ def test_missing_dir(test_data_dir, tmpdir):
     assert examples_dataset.id == f'{name}/'
 
     out_path = tmpdir / 'crate_read_out'
+    with pytest.raises(OSError):
+        crate.write(out_path)
+
+    # Two options to get a writable crate
+
+    # 1. Set the source to None (will create an empty dir)
+    examples_dataset.source = None
+    crate.write(out_path)
+    assert (out_path / name).is_dir()
+
+    shutil.rmtree(out_path)
+
+    # 2. Provide an existing source
+    source = tmpdir / "source"
+    source.mkdir()
+    examples_dataset.source = source
     crate.write(out_path)
-    assert not (out_path / 'README.txt').exists()
+    assert (out_path / name).is_dir()
 
 
+@pytest.mark.filterwarnings("ignore")
 def test_missing_file(test_data_dir, tmpdir):
     crate_dir = test_data_dir / 'read_crate'
     name = 'test_file_galaxy.txt'
@@ -335,9 +352,26 @@ def test_missing_file(test_data_dir, tmpdir):
     assert test_file.id == name
 
     out_path = tmpdir / 'crate_read_out'
+    with pytest.raises(OSError):
+        crate.write(out_path)
+
+    # Two options to get a writable crate
+
+    # 1. Set the source to None (file will still be missing in the copy)
+    test_file.source = None
     crate.write(out_path)
     assert not (out_path / name).exists()
 
+    shutil.rmtree(out_path)
+
+    # 2. Provide an existing source
+    source = tmpdir / "source.txt"
+    text = "foo\nbar\n"
+    source.write_text(text)
+    test_file.source = source
+    crate.write(out_path)
+    assert (out_path / name).read_text() == text
+
 
 def test_generic_data_entity(tmpdir):
     rc_id = "#collection"
diff --git a/test/test_write.py b/test/test_write.py
index b806ae5..15a5244 100644
--- a/test/test_write.py
+++ b/test/test_write.py
@@ -301,23 +301,36 @@ def test_remote_uri_exceptions(tmpdir):
     # no error on Windows, or on Linux as root, so we don't use pytest.raises
 
 
-@pytest.mark.parametrize("fetch_remote,validate_url", [(False, False), (False, True), (True, False), (True, True)])
-def test_missing_source(test_data_dir, tmpdir, fetch_remote, validate_url):
+@pytest.mark.filterwarnings("ignore")
+@pytest.mark.parametrize("what", ["file", "dataset"])
+def test_missing_source(test_data_dir, tmpdir, what):
     path = test_data_dir / uuid.uuid4().hex
-    args = {"fetch_remote": fetch_remote, "validate_url": validate_url}
 
     crate = ROCrate()
-    file_ = crate.add_file(path, **args)
-    assert file_ is crate.dereference(path.name)
-    out_path = tmpdir / 'ro_crate_out_1'
-    crate.write(out_path)
-    assert not (out_path / path.name).exists()
+    entity = getattr(crate, f"add_{what}")(path)
+    assert entity is crate.dereference(path.name)
+    crate_dir = tmpdir / 'ro_crate_out_1'
+    with pytest.raises(OSError):
+        crate.write(crate_dir)
 
     crate = ROCrate()
-    file_ = crate.add_file(path, path.name, **args)
-    assert file_ is crate.dereference(path.name)
-    out_path = tmpdir / 'ro_crate_out_2'
-    assert not (out_path / path.name).exists()
+    entity = getattr(crate, f"add_{what}")(path, path.name)
+    assert entity is crate.dereference(path.name)
+    crate_dir = tmpdir / 'ro_crate_out_2'
+    with pytest.raises(OSError):
+        crate.write(crate_dir)
+
+    crate = ROCrate()
+    entity = getattr(crate, f"add_{what}")(None, path.name)
+    assert entity is crate.dereference(path.name)
+    crate_dir = tmpdir / 'ro_crate_out_3'
+    crate.write(crate_dir)
+    out_path = crate_dir / path.name
+    if what == "file":
+        assert not out_path.exists()
+    else:
+        assert out_path.is_dir()
+        assert not any(out_path.iterdir())
 
 
 @pytest.mark.parametrize("fetch_remote,validate_url", [(False, False), (False, True), (True, False), (True, True)])
@@ -336,12 +349,15 @@ def test_no_source_no_dest(test_data_dir, fetch_remote, validate_url):
 
 def test_dataset(test_data_dir, tmpdir):
     crate = ROCrate()
-    path = test_data_dir / "a" / "b"
-    d1 = crate.add_dataset(path)
+    path_a_b = test_data_dir / "a" / "b"
+    path_c = test_data_dir / "c"
+    for p in path_a_b, path_c:
+        p.mkdir(parents=True)
+    d1 = crate.add_dataset(path_a_b)
     assert crate.dereference("b") is d1
-    d2 = crate.add_dataset(path, "a/b")
+    d2 = crate.add_dataset(path_a_b, "a/b")
     assert crate.dereference("a/b") is d2
-    d_from_str = crate.add_dataset(str(test_data_dir / "c"))
+    d_from_str = crate.add_dataset(str(path_c))
     assert crate.dereference("c") is d_from_str
 
     out_path = tmpdir / 'ro_crate_out'

Fix behavior wrt "missing" data entities
After the merge of #75, we allow data entities whose `@id` does not map to an existing file or directory, even if local. While this adds flexibility for use cases like #73, such crates are not supported by other libraries. Specifically, I got reports of crates generated with ro-crate-py that lead to errors when submission to WorkflowHub is attempted. The main issue is that it's too easy to add a "missing" data entity:

```python
crate.add_file("/this/does/not/exist")  # Adds a File entity with @id = "exist"
```

This means it can be done involuntarily, by simply mistyping the path. It should be harder to do so (advanced usage), and we should issue a warning when it's done.
test/test_read.py::test_missing_dir test/test_read.py::test_missing_file test/test_write.py::test_missing_source[file] test/test_write.py::test_missing_source[dataset]
test/test_read.py::test_crate_dir_loading[False-False] test/test_read.py::test_crate_dir_loading[True-False] test/test_read.py::test_crate_dir_loading[True-True] test/test_read.py::test_legacy_crate test/test_read.py::test_bad_crate test/test_read.py::test_init[False] test/test_read.py::test_init[True] test/test_read.py::test_exclude test/test_read.py::test_init_preview[False-False] test/test_read.py::test_init_preview[False-True] test/test_read.py::test_init_preview[True-False] test/test_read.py::test_init_preview[True-True] test/test_read.py::test_no_parts test/test_read.py::test_extra_data[False] test/test_read.py::test_extra_data[True] test/test_read.py::test_generic_data_entity test/test_read.py::test_root_conformsto test/test_read.py::test_multi_type_context_entity test/test_write.py::test_file_writing[False-False] test/test_write.py::test_file_writing[False-True] test/test_write.py::test_file_writing[True-False] test/test_write.py::test_file_writing[True-True] test/test_write.py::test_in_mem_stream[BytesIO] test/test_write.py::test_in_mem_stream[StringIO] test/test_write.py::test_remote_uri[False-False-False] test/test_write.py::test_remote_uri[False-False-True] test/test_write.py::test_remote_uri[False-True-False] test/test_write.py::test_remote_uri[False-True-True] test/test_write.py::test_remote_uri[True-False-False] test/test_write.py::test_remote_uri[True-False-True] test/test_write.py::test_remote_uri[True-True-False] test/test_write.py::test_remote_uri[True-True-True] test/test_write.py::test_file_uri test/test_write.py::test_looks_like_file_uri test/test_write.py::test_ftp_uri[False] test/test_write.py::test_remote_dir[False-False] test/test_write.py::test_remote_dir[False-True] test/test_write.py::test_remote_dir[True-False] test/test_write.py::test_remote_dir[True-True] test/test_write.py::test_remote_uri_exceptions test/test_write.py::test_stringio_no_dest[False-False] test/test_write.py::test_stringio_no_dest[False-True] test/test_write.py::test_stringio_no_dest[True-False] test/test_write.py::test_stringio_no_dest[True-True] test/test_write.py::test_no_source_no_dest[False-False] test/test_write.py::test_no_source_no_dest[False-True] test/test_write.py::test_no_source_no_dest[True-False] test/test_write.py::test_no_source_no_dest[True-True] test/test_write.py::test_dataset test/test_write.py::test_no_parts test/test_write.py::test_no_zip_in_zip