diff --git a/lib/spack/spack/fetch_strategy.py b/lib/spack/spack/fetch_strategy.py
index aaec9a40cd0..bd7331e1a13 100644
--- a/lib/spack/spack/fetch_strategy.py
+++ b/lib/spack/spack/fetch_strategy.py
@@ -521,7 +521,7 @@ def expand(self):
                 "Failed on expand() for URL %s" % self.url)
 
         if not self.extension:
-            self.extension = extension(self.archive_file)
+            self.extension = extension(self.url)
 
         if self.stage.expanded:
             tty.debug('Source already staged to %s' % self.stage.source_path)
diff --git a/lib/spack/spack/util/compression.py b/lib/spack/spack/util/compression.py
index 9a8eda0c913..18c0a9ea5e2 100644
--- a/lib/spack/spack/util/compression.py
+++ b/lib/spack/spack/util/compression.py
@@ -246,7 +246,7 @@ def _7zip(archive_file):
     return outfile
 
 
-def decompressor_for(path, ext=None):
+def decompressor_for(path, ext):
     """Returns a function pointer to appropriate decompression
     algorithm based on extension type.
 
@@ -254,9 +254,6 @@ def decompressor_for(path, ext=None):
         path (str): path of the archive file requiring decompression
         ext (str): Extension of archive file
     """
-    if not ext:
-        ext = extension(path)
-
     if not allowed_archive(ext):
         raise CommandNotFoundError("Cannot extract archive, \
 unrecognized file extension: '%s'" % ext)