diff --git a/var/spack/repos/builtin/packages/ffmpeg/package.py b/var/spack/repos/builtin/packages/ffmpeg/package.py
index ada37e80350..75185fc5661 100644
--- a/var/spack/repos/builtin/packages/ffmpeg/package.py
+++ b/var/spack/repos/builtin/packages/ffmpeg/package.py
@@ -111,7 +111,7 @@ def libs(self):
     @property
     def headers(self):
         headers = find_all_headers(self.prefix.include)
-        headers.directories = self.prefix.include
+        headers.directories = [self.prefix.include]
         return headers
 
     def enable_or_disable_meta(self, variant, options):
diff --git a/var/spack/repos/builtin/packages/gnutls/package.py b/var/spack/repos/builtin/packages/gnutls/package.py
index e258cc5ad5a..875a2c52bd8 100644
--- a/var/spack/repos/builtin/packages/gnutls/package.py
+++ b/var/spack/repos/builtin/packages/gnutls/package.py
@@ -94,5 +94,5 @@ def configure_args(self):
     @property
     def headers(self):
         headers = find_all_headers(self.prefix.include)
-        headers.directories = self.prefix.include
+        headers.directories = [self.prefix.include]
         return headers
diff --git a/var/spack/repos/builtin/packages/py-torch/package.py b/var/spack/repos/builtin/packages/py-torch/package.py
index 71b71fe210f..54056f177ff 100644
--- a/var/spack/repos/builtin/packages/py-torch/package.py
+++ b/var/spack/repos/builtin/packages/py-torch/package.py
@@ -208,7 +208,7 @@ def headers(self):
             self.prefix, self.spec['python'].package.site_packages_dir,
             'torch', 'include')
         headers = find_all_headers(root)
-        headers.directories = root
+        headers.directories = [root]
         return headers
 
     def setup_build_environment(self, env):