diff --git a/.gitattributes b/.gitattributes
index 03e300a8e6..e2ce130409 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -8228,6 +8228,7 @@ tests/webtbf/tw12365b.pp svneol=native#text/plain
 tests/webtbf/tw1238.pp svneol=native#text/plain
 tests/webtbf/tw1251a.pp svneol=native#text/plain
 tests/webtbf/tw1270.pp svneol=native#text/plain
+tests/webtbf/tw12933.pp svneol=native#text/plain
 tests/webtbf/tw1306.pp svneol=native#text/plain
 tests/webtbf/tw1316.pp svneol=native#text/plain
 tests/webtbf/tw1328.pp svneol=native#text/plain
diff --git a/compiler/htypechk.pas b/compiler/htypechk.pas
index 465f967048..2ed37f0651 100644
--- a/compiler/htypechk.pas
+++ b/compiler/htypechk.pas
@@ -1495,8 +1495,13 @@ implementation
               { if they are objects              }
               if (def_from.typ=objectdef) and
                  (
-                  not(m_delphi in current_settings.modeswitches) or
                   (
+                   not(m_delphi in current_settings.modeswitches) and
+                   (tobjectdef(def_from).objecttype in [odt_object,odt_class]) and
+                   (tobjectdef(def_to).objecttype in [odt_object,odt_class])
+                  ) or
+                  (
+                   (m_delphi in current_settings.modeswitches) and
                    (tobjectdef(def_from).objecttype=odt_object) and
                    (tobjectdef(def_to).objecttype=odt_object)
                   )
diff --git a/tests/webtbf/tw12933.pp b/tests/webtbf/tw12933.pp
new file mode 100644
index 0000000000..2f193f16b5
--- /dev/null
+++ b/tests/webtbf/tw12933.pp
@@ -0,0 +1,20 @@
+{ %fail }
+
+{$mode objfpc}
+
+type
+  ta = interface
+  end;
+
+  tb = interface(ta)
+  end;
+
+procedure test(var a: ta);
+begin
+end;
+
+var
+  b: tb;
+begin
+  test(b);
+end.