diff --git a/cpm_kernels/library/base.py b/cpm_kernels/library/base.py
index ad319cf..6d82373 100644
--- a/cpm_kernels/library/base.py
+++ b/cpm_kernels/library/base.py
@@ -11,7 +11,8 @@
 def lookup_dll(prefix):
     paths = os.environ.get("PATH", "").split(os.pathsep)
     for path in paths:
-        if not os.path.exists(path):
+        # if not dir, then ignore. or os.listdir(path) will raise error.
+        if not os.path.exists(path) or not os.path.isdir(path):
             continue
         for name in os.listdir(path):
             if name.startswith(prefix) and name.lower().endswith(".dll"):