diff --git a/src/zope/app/apidoc/utilities.py b/src/zope/app/apidoc/utilities.py
index 185e3987..1b353f24 100644
--- a/src/zope/app/apidoc/utilities.py
+++ b/src/zope/app/apidoc/utilities.py
@@ -53,7 +53,7 @@ def relativizePath(path):
         return path
     longest_matching_path = max(matching_paths, key=len)
     common_prefix = os.path.commonprefix([longest_matching_path, path])
-    return path.replace(common_prefix, 'Zope3')
+    return path.replace(common_prefix, 'Zope3') if common_prefix else path
 
 def truncateSysPath(path):
     """Remove the system path prefix from the path."""
@@ -62,7 +62,7 @@ def truncateSysPath(path):
         return path
     longest_matching_path = max(matching_paths, key=len)
     common_prefix = os.path.commonprefix([longest_matching_path, path])
-    return path.replace(common_prefix, '')[1:]
+    return path.replace(common_prefix, '')[1:] if common_prefix else path
 
 @implementer(IReadContainer)
 class ReadContainerBase(object):