diff --git a/examples/tutorial/devices.py b/examples/tutorial/devices.py
index c9f8977..46fcd27 100644
--- a/examples/tutorial/devices.py
+++ b/examples/tutorial/devices.py
@@ -142,7 +142,7 @@ def waveguide(
         left=Port((-extent, 0), rotation=0, ptype='pcwg'),
         right=Port((extent, 0), rotation=pi, ptype='pcwg'),
         )
-    pat2dev(pat)
+    dev2pat(pat)
     print(pat)
     return pat
 
@@ -183,7 +183,7 @@ def bend(
                     extent * numpy.sqrt(3) / 2),
                    rotation=pi * 4 / 3, ptype='pcwg'),
         )
-    pat2dev(pat)
+    dev2pat(pat)
     return pat
 
 
@@ -222,7 +222,7 @@ def y_splitter(
         'bot': Port((extent / 2, -extent * numpy.sqrt(3) / 2), rotation=pi * 2 / 3, ptype='pcwg'),
         }
 
-    pat2dev(pat)
+    dev2pat(pat)
     return pat
 
 
@@ -311,7 +311,7 @@ def main(interactive: bool = True) -> None:
     # We can also add text labels for our circuit's ports.
     #   They will appear at the uppermost hierarchy level, while the individual
     # device ports will appear further down, in their respective cells.
-    pat2dev(circ.pattern)
+    dev2pat(circ.pattern)
 
     # Add the pattern into our library
     lib['my_circuit'] = circ.pattern
@@ -319,7 +319,7 @@ def main(interactive: bool = True) -> None:
     # Check if we forgot to include any patterns... ooops!
     if dangling := lib.dangling_refs():
         print('Warning: The following patterns are referenced, but not present in the'
-              f'library! {dangling}')
+              f' library! {dangling}')
         print('We\'ll solve this by merging in shape_lib, which contains those shapes...')
 
         lib.add(shape_lib)
diff --git a/masque/abstract.py b/masque/abstract.py
index 04ce88b..9b06d1c 100644
--- a/masque/abstract.py
+++ b/masque/abstract.py
@@ -1,9 +1,12 @@
-from typing import Dict
+from typing import Dict, TypeVar
 #from typing import Union, Optional, MutableMapping, TYPE_CHECKING
 import copy
 import logging
 
+from numpy.typing import ArrayLike
+
 #from .pattern import Pattern
+from .ref import Ref
 from .ports import PortList, Port
 
 #if TYPE_CHECKING:
diff --git a/masque/builder/builder.py b/masque/builder/builder.py
index e0b6c8f..c33a3ef 100644
--- a/masque/builder/builder.py
+++ b/masque/builder/builder.py
@@ -207,7 +207,7 @@ class Builder(PortList):
                 names.
         """
         if library is None:
-            if hasattr(source, 'library') and isinstance(source, MutableLibrary):
+            if hasattr(source, 'library') and isinstance(source.library, MutableLibrary):
                 library = source.library
             else:
                 raise BuildError('No library provided (and not present in `source.library`')
diff --git a/masque/builder/port_utils.py b/masque/builder/port_utils.py
index 75ad2b7..5a1d5e9 100644
--- a/masque/builder/port_utils.py
+++ b/masque/builder/port_utils.py
@@ -53,7 +53,9 @@ def dev2pat(pattern: Pattern, layer: layer_t) -> Pattern:
 def pat2dev(
         layers: Sequence[layer_t],
         library: Mapping[str, Pattern],
-        pattern: Pattern,
+        pattern: Pattern,               # Pattern is good since we don't want to do library[name] to avoid infinite recursion.
+                                        # LazyLibrary protects against library[ref.target] causing a circular lookup.
+                                        # For others, maybe check for cycles up front? TODO
         name: Optional[str] = None,
         max_depth: int = 999_999,
         skip_subcells: bool = True,
@@ -94,20 +96,24 @@ def pat2dev(
     if (skip_subcells and pattern.ports) or max_depth == 0:
         return pattern
 
-    # Load ports for all subpatterns
-    for target in set(rr.target for rr in pat.refs):
+    # Load ports for all subpatterns, and use any we find
+    found_ports = False
+    for target in set(rr.target for rr in pattern.refs):
         pp = pat2dev(
             layers=layers,
             library=library,
             pattern=library[target],
             name=target,
-            max_depth=max_depth-1,
+            max_depth=max_depth - 1,
             skip_subcells=skip_subcells,
             blacklist=blacklist + {name},
             )
         found_ports |= bool(pp.ports)
 
-    for ref in pat.refs:
+    if not found_ports:
+        return pattern
+
+    for ref in pattern.refs:
         aa = library.abstract(ref.target)
         if not aa.ports:
             continue
diff --git a/masque/library.py b/masque/library.py
index b9ac8de..bd2857d 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -713,11 +713,13 @@ class LazyLibrary(MutableLibrary):
     """
     dict: Dict[str, Callable[[], 'Pattern']]
     cache: Dict[str, 'Pattern']
+    _lookups_in_progress: Set[str]
     enable_cache: bool = True
 
     def __init__(self) -> None:
         self.dict = {}
         self.cache = {}
+        self._lookups_in_progress = set()
 
     def __setitem__(self, key: str, value: Callable[[], 'Pattern']) -> None:
         self.dict[key] = value
@@ -735,8 +737,17 @@ class LazyLibrary(MutableLibrary):
             logger.debug(f'found {key} in cache')
             return self.cache[key]
 
+        if key in self._lookups_in_progress:
+            raise LibraryError(
+                f'Detected multiple simultaneous lookups of "{key}".\n'
+                'This may be caused by an invalid (cyclical) reference, or buggy code.\n'
+                'If you are lazy-loading a file, try a non-lazy load and check for refernce cycles.'        # TODO give advice on finding cycles
+                )
+
+        self._lookups_in_progress.add(key)
         func = self.dict[key]
         pat = func()
+        self._lookups_in_progress.remove(key)
         self.cache[key] = pat
         return pat