diff --git a/docs/sphinx/source/changelog.rst b/docs/sphinx/source/changelog.rst
index d29155dc..8b5fe59f 100644
--- a/docs/sphinx/source/changelog.rst
+++ b/docs/sphinx/source/changelog.rst
@@ -1,5 +1,6 @@
 RdTools Change Log
 ==================
+.. include:: changelog/v2.1.8.rst
 .. include:: changelog/v2.1.7.rst
 .. include:: changelog/v2.1.6.rst
 .. include:: changelog/v2.1.5.rst
diff --git a/docs/sphinx/source/changelog/v2.1.8.rst b/docs/sphinx/source/changelog/v2.1.8.rst
new file mode 100644
index 00000000..ca9b663a
--- /dev/null
+++ b/docs/sphinx/source/changelog/v2.1.8.rst
@@ -0,0 +1,11 @@
+**************************
+v2.1.8 (November 17, 2023)
+**************************
+
+Bug fixes
+---------
+* Fix a bug in parallel computing environments (:issue:`388` :pull:`399`)
+
+Contributors
+------------
+* Martin Springer (:ghuser:`martin-springer`)
\ No newline at end of file
diff --git a/rdtools/normalization.py b/rdtools/normalization.py
index df082100..8629c237 100644
--- a/rdtools/normalization.py
+++ b/rdtools/normalization.py
@@ -380,7 +380,7 @@ def irradiance_rescale(irrad, irrad_sim, max_iterations=100,
     '''
 
     if method == 'iterative':
-        def _rmse(fact):
+        def _rmse(fact, filt):
             """
             Calculates RMSE with a given rescale fact(or) according to global
             filt(er)
@@ -392,10 +392,9 @@ def _rmse(fact):
 
         def _single_rescale(irrad, irrad_sim, guess):
             "Optimizes rescale factor once"
-            global filt
             csi = irrad / (guess * irrad_sim)  # clear sky index
             filt = (csi >= 0.8) & (csi <= 1.2) & (irrad > 200)
-            min_result = minimize(_rmse, guess, method='Nelder-Mead')
+            min_result = minimize(_rmse, guess, (filt), method='Nelder-Mead')
 
             factor = min_result['x'][0]
             return factor