diff --git a/src/mdevaluate/checksum.py b/src/mdevaluate/checksum.py
index 1c0d745..a6d7f06 100755
--- a/src/mdevaluate/checksum.py
+++ b/src/mdevaluate/checksum.py
@@ -102,8 +102,20 @@ def checksum(*args, csum=None, _seen=None):
         _seen.add(obj_id)
 
         if hasattr(arg, "__checksum__"):
-            logger.debug("Checksum via __checksum__: %s", str(arg))
-            csum.update(str(arg.__checksum__()).encode())
+            method = getattr(arg, "__checksum__")
+            if callable(method) and not isinstance(arg, type):
+                logger.debug("Checksum via __checksum__: %s", str(arg))
+                csum.update(str(method()).encode())
+            elif isinstance(arg, type):
+                try:
+                    src = inspect.getsource(arg)
+                    csum.update(strip_comments(src).encode())
+                    logger.debug("Checksum via class source for %s", arg.__name__)
+                except (OSError, TypeError):
+                    csum.update(arg.__name__.encode())
+                    logger.debug("Checksum via class name for %s", arg.__name__)
+            else:
+                logger.debug("Skipping unbound __checksum__ on %s", type(arg))
         elif isinstance(arg, bytes):
             csum.update(arg)
         elif isinstance(arg, str):