diff --git a/ogs6py/classes/processes.py b/ogs6py/classes/processes.py
index f477319..a6d3703 100644
--- a/ogs6py/classes/processes.py
+++ b/ogs6py/classes/processes.py
@@ -87,10 +87,19 @@ def set_process(self, **args):
                     self.populate_tree(self.process_baseentries["darcy_gravity"],
                             "g",text = str(entry))
         if "specific_body_force" in args:
-            self.populate_tree(self.process, "specific_body_force", text=" ".join(str(x) for x in args['specific_body_force']))
+            if isinstance(args["specific_body_force"], list):
+                self.populate_tree(self.process, "specific_body_force",
+                                   text=" ".join(str(x) for x in args['specific_body_force']))
+            else:
+                self.populate_tree(self.process, "specific_body_force",
+                                   text=args["specific_body_force"])
         for key, value in args.items():
-            if isinstance(value, str):
-                self.populate_tree(self.process, key, text=value)
+            if key not in ["darcy_gravity", "specific_body_force"]:
+                if isinstance(value, str):
+                    self.populate_tree(self.process, key, text=value)
+                else:
+                    raise RuntimeError(f"{key} is not of type string")
+
 
 
     def set_constitutive_relation(self, **args):