diff --git a/lib/spack/spack/fetch_strategy.py b/lib/spack/spack/fetch_strategy.py
index 30e6823207e..f1f787f4c7f 100644
--- a/lib/spack/spack/fetch_strategy.py
+++ b/lib/spack/spack/fetch_strategy.py
@@ -276,7 +276,8 @@ def fetch(self):
         # Check if we somehow got an HTML file rather than the archive we
         # asked for.  We only look at the last content type, to handle
         # redirects properly.
-        content_types = re.findall(r'Content-Type:[^\r\n]+', headers)
+        content_types = re.findall(r'Content-Type:[^\r\n]+', headers,
+                                   flags=re.IGNORECASE)
         if content_types and 'text/html' in content_types[-1]:
             tty.warn("The contents of ",
                      (self.archive_file if self.archive_file is not None