diff --git a/maxray/__init__.py b/maxray/__init__.py
index 1624151..718c194 100644
--- a/maxray/__init__.py
+++ b/maxray/__init__.py
@@ -1,6 +1,7 @@
 from .transforms import recompile_fn_with_transform, NodeContext
 
 import inspect
+from weakref import ref, WeakSet
 from contextvars import ContextVar
 from contextlib import contextmanager
 from dataclasses import dataclass
@@ -101,7 +102,9 @@ def descend_allowed(x, ctx: NodeContext):
 _GLOBAL_WRITER_ACTIVE_FLAG = ContextVar("writer_active (global)", default=False)
 
 # We don't want to recompile the same function over and over - so our cache needs to be global
+# TODO: add cache by source code location?
 _MAXRAY_FN_CACHE = dict()
+_MAXRAY_FN_FAILED_CACHE = WeakSet()
 
 
 def callable_allowed_for_transform(x, ctx: NodeContext):
@@ -179,6 +182,8 @@ def _maxray_walker_handler(x, ctx: NodeContext):
     # 1b. normal functions or bound methods or method descriptors like @classmethod and @staticmethod
     elif callable_allowed_for_transform(x, ctx):
         # We can only cache functions - as caching invokes __hash__, which may fail badly on incompletely-initialised class instances w/ __call__ methods, like torch._ops.OpOverload
+        if x in _MAXRAY_FN_FAILED_CACHE:
+            pass
         if x in _MAXRAY_FN_CACHE:
             x = _MAXRAY_FN_CACHE[x]
         elif not descend_allowed(x, ctx):
@@ -219,7 +224,7 @@ def _maxray_walker_handler(x, ctx: NodeContext):
 
                 case Err(e):
                     # Cache failures
-                    _MAXRAY_FN_CACHE[x] = x
+                    _MAXRAY_FN_FAILED_CACHE.add(x)
                     # Errors in functions that have been recursively compiled are less important
                     logger.warning(f"Failed to transform in walker handler: {e}")
 
diff --git a/maxray/transforms.py b/maxray/transforms.py
index a110506..b4220ac 100644
--- a/maxray/transforms.py
+++ b/maxray/transforms.py
@@ -509,6 +509,11 @@ def recompile_fn_with_transform(
                 f"Non-existent source file ({sourcefile}) for function {get_fn_name(source_fn)}"
             )
 
+        if module is None:
+            return Err(
+                f"Non-existent source module `{getattr(source_fn, '__module__', None)}` for function {get_fn_name(source_fn)}"
+            )
+
         fn_ast = ast.parse(source)
     except OSError:
         return Err(f"No source code for function {get_fn_name(source_fn)}")