diff --git a/pyproject.toml b/pyproject.toml
index 1dc9ac71..01c167e3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -39,7 +39,7 @@ classifiers = [
 
 requires-python = ">=3.9"
 dependencies = [
-    'oemof.solph',
+    'oemof.solph <= 0.5.5',
     'matplotlib',
     'pvlib <= 0.9.0',
     'numpy >= 1.16.5',