diff --git a/src/pystencilssfg/ir/source_components.py b/src/pystencilssfg/ir/source_components.py
index c2869290c6d4f893bea5bf67920b038e26910646..4398938327242e8e1c1fb2cfc7787c72fa0d3138 100644
--- a/src/pystencilssfg/ir/source_components.py
+++ b/src/pystencilssfg/ir/source_components.py
@@ -168,7 +168,7 @@ class SfgKernelHandle:
 
         for param in self._parameters:
             if param.wrapped.is_field_parameter:
-                self._fields |= param.wrapped.fields
+                self._fields |= set(param.wrapped.fields)
             else:
                 self._scalar_params.add(param)