diff --git a/CHANGELOG.md b/CHANGELOG.md
index 81dd383..cf74bf3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,11 @@
 This file logs the changes that are actually interesting to users (new features,
 changed functionality, fixed bugs).
 
+## Version 1.6 (in development)
+
+- Support linked collections used as input in a Geometry Nodes modifier.
+
+
 ## Version 1.5.1 (2021-07-22)
 
 - Add log warning if SegmentationFault caused by dereferencing invalid pointer is silenced when strict_pointer_mode is turned off.
diff --git a/blender_asset_tracer/cdefs.py b/blender_asset_tracer/cdefs.py
index 38fbbca..bbc9bd1 100644
--- a/blender_asset_tracer/cdefs.py
+++ b/blender_asset_tracer/cdefs.py
@@ -61,6 +61,17 @@ OB_DUPLIGROUP = 1 << 8
 PTCACHE_DISK_CACHE = 64
 PTCACHE_EXTERNAL = 512
 
+# DNA_ID_types.h
+IDP_STRING = 0
+IDP_INT = 1
+IDP_FLOAT = 2
+IDP_ARRAY = 5
+IDP_GROUP = 6
+IDP_ID = 7
+IDP_DOUBLE = 8
+IDP_IDPARRAY = 9
+IDP_NUMTYPES = 10
+
 # BKE_pointcache.h
 PTCACHE_FILE_PTCACHE = 0
 PTCACHE_FILE_OPENVDB = 1
diff --git a/blender_asset_tracer/trace/expanders.py b/blender_asset_tracer/trace/expanders.py
index bce34c0..6b8da42 100644
--- a/blender_asset_tracer/trace/expanders.py
+++ b/blender_asset_tracer/trace/expanders.py
@@ -103,6 +103,28 @@ def _expand_generic_nodetree(block: blendfile.BlendFileBlock):
         yield node.get_pointer(b"id")
 
 
+def _expand_generic_idprops(block: blendfile.BlendFileBlock):
+    """Yield ID datablocks and their libraries referenced from ID properties."""
+
+    # TODO(@sybren): this code is very crude, and happens to work on ID
+    # properties of Geometry Nodes modifiers, which is what it was written for.
+    # It should probably be rewritten to properly iterate over & recurse into
+    # all groups.
+    settings_props = block.get_pointer((b"settings", b"properties"))
+    if not settings_props:
+        return
+
+    subprops = settings_props.get_pointer((b"data", b"group", b"first"))
+    for idprop in iterators.listbase(subprops):
+        if idprop[b"type"] != cdefs.IDP_ID:
+            continue
+        id_datablock = idprop.get_pointer((b"data", b"pointer"))
+        if not id_datablock:
+            continue
+        yield id_datablock
+        yield id_datablock.get_pointer(b"lib")
+
+
 def _expand_generic_nodetree_id(block: blendfile.BlendFileBlock):
     block_ntree = block.get_pointer(b"nodetree", None)
     if block_ntree is not None:
@@ -251,6 +273,7 @@ def _expand_object(block: blendfile.BlendFileBlock):
         # Currently only node groups are supported. If the support should expand
         # to more types, something more intelligent than this should be made.
         if mod_type == cdefs.eModifierType_Nodes:
+            yield from _expand_generic_idprops(block_mod)
             yield block_mod.get_pointer(b"node_group")
 
 
diff --git a/blender_asset_tracer/trace/file2blocks.py b/blender_asset_tracer/trace/file2blocks.py
index 8604c52..63074d4 100644
--- a/blender_asset_tracer/trace/file2blocks.py
+++ b/blender_asset_tracer/trace/file2blocks.py
@@ -110,11 +110,7 @@ class BlockIterator:
                 self.to_visit.put(lib)
                 continue
 
-            if limit_to:
-                # We're limiting the blocks, so we have to expand them to make
-                # sure we don't miss anything. Otherwise we're yielding the
-                # entire file anyway, and no expansion is necessary.
-                self._queue_dependencies(block)
+            self._queue_dependencies(block)
             self.blocks_yielded.add((bpath, block.addr_old))
             yield block
 
@@ -167,6 +163,7 @@ class BlockIterator:
 
     def _queue_dependencies(self, block: blendfile.BlendFileBlock):
         for block in expanders.expand_block(block):
+            assert isinstance(block, blendfile.BlendFileBlock), "unexpected %r" % block
             self.to_visit.put(block)
 
 
diff --git a/tests/blendfiles/geometry-nodes-2/lib_trash.blend b/tests/blendfiles/geometry-nodes-2/lib_trash.blend
new file mode 100644
index 0000000..c021619
Binary files /dev/null and b/tests/blendfiles/geometry-nodes-2/lib_trash.blend differ
diff --git a/tests/blendfiles/geometry-nodes-2/set_file.blend b/tests/blendfiles/geometry-nodes-2/set_file.blend
new file mode 100644
index 0000000..6d13c09
Binary files /dev/null and b/tests/blendfiles/geometry-nodes-2/set_file.blend differ
diff --git a/tests/blendfiles/geometry-nodes-2/shot_file.blend b/tests/blendfiles/geometry-nodes-2/shot_file.blend
new file mode 100644
index 0000000..025d169
Binary files /dev/null and b/tests/blendfiles/geometry-nodes-2/shot_file.blend differ
diff --git a/tests/test_tracer.py b/tests/test_tracer.py
index 1ea7820..caa744d 100644
--- a/tests/test_tracer.py
+++ b/tests/test_tracer.py
@@ -436,6 +436,34 @@ class DepsTest(AbstractTracerTest):
             },
         )
 
+    def test_geometry_nodes_modifier_input(self):
+        """Test linked collection as input to geom nodes modifier.
+
+        Here a Geometry Nodes modifier references a collection that is not
+        instanced into the scene, which caused it to be missed.
+        """
+        self.assert_deps(
+            "geometry-nodes-2/shot_file.blend",
+            {
+                b"LIset_file.blend": Expect(
+                    type="Library",
+                    full_field="name[1024]",
+                    dirname_field=None,
+                    basename_field=None,
+                    asset_path=b"//set_file.blend",
+                    is_sequence=False,
+                ),
+                b"LIlib_trash.blend": Expect(
+                    type="Library",
+                    full_field="name[1024]",
+                    dirname_field=None,
+                    basename_field=None,
+                    asset_path=b"//lib_trash.blend",
+                    is_sequence=False,
+                ),
+            },
+        )
+
     def test_usage_abspath(self):
         deps = [
             dep