diff --git a/blueprintcompiler/gir.py b/blueprintcompiler/gir.py
index 7c5a67d..32eca16 100644
--- a/blueprintcompiler/gir.py
+++ b/blueprintcompiler/gir.py
@@ -257,9 +257,11 @@ class Class(GirNode, GirType):
             return True
         elif self.parent and self.parent.assignable_to(other):
             return True
-        elif other in self.implements:
-            return True
         else:
+            for iface in self.implements:
+                if self.get_containing(Namespace).lookup_type(iface).assignable_to(other):
+                    return True
+
             return False
 
 
diff --git a/tests/sample_errors/does_not_implement.blp b/tests/sample_errors/does_not_implement.blp
new file mode 100644
index 0000000..31bbf06
--- /dev/null
+++ b/tests/sample_errors/does_not_implement.blp
@@ -0,0 +1,7 @@
+using Gtk 4.0;
+
+Label label {}
+
+DropDown {
+  model: label;
+}
diff --git a/tests/sample_errors/does_not_implement.err b/tests/sample_errors/does_not_implement.err
new file mode 100644
index 0000000..ad64f1c
--- /dev/null
+++ b/tests/sample_errors/does_not_implement.err
@@ -0,0 +1 @@
+6,10,5,Cannot assign Gtk.Label to Gio.ListModel
diff --git a/tests/samples/string_list.blp b/tests/samples/string_list.blp
index 2f1198c..91af385 100644
--- a/tests/samples/string_list.blp
+++ b/tests/samples/string_list.blp
@@ -1,8 +1,12 @@
 using Gtk 4.0;
 
-StringList {
+StringList greetings {
   strings [
     "Hello, world!",
     _("Hello!"),
   ]
 }
+
+Gtk.DropDown {
+  model: greetings;
+}
diff --git a/tests/samples/string_list.ui b/tests/samples/string_list.ui
index 02078a0..156bc09 100644
--- a/tests/samples/string_list.ui
+++ b/tests/samples/string_list.ui
@@ -1,10 +1,13 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
   <requires lib="gtk" version="4.0"/>
-  <object class="GtkStringList">
+  <object class="GtkStringList" id="greetings">
     <items>
       <item>Hello, world!</item>
       <item translatable="true">Hello!</item>
     </items>
   </object>
+  <object class="GtkDropDown">
+    <property name="model">greetings</property>
+  </object>
 </interface>
diff --git a/tests/test_samples.py b/tests/test_samples.py
index 572b706..70553b4 100644
--- a/tests/test_samples.py
+++ b/tests/test_samples.py
@@ -144,6 +144,7 @@ class TestSamples(unittest.TestCase):
         self.assert_sample_error("class_assign")
         self.assert_sample_error("class_dne")
         self.assert_sample_error("consecutive_unexpected_tokens")
+        self.assert_sample_error("does_not_implement")
         self.assert_sample_error("duplicate_obj_id")
         self.assert_sample_error("enum_member_dne")
         self.assert_sample_error("filters_in_non_file_filter")