diff --git a/src/subscript/merge_rft_ertobs/merge_rft_ertobs.py b/src/subscript/merge_rft_ertobs/merge_rft_ertobs.py
index 078d07aa2..f391d8cc8 100644
--- a/src/subscript/merge_rft_ertobs/merge_rft_ertobs.py
+++ b/src/subscript/merge_rft_ertobs/merge_rft_ertobs.py
@@ -223,8 +223,11 @@ def merge_rft_ertobs(gendatacsv: str, obsdir: str) -> pd.DataFrame:
 
     # For each simulated well, look up
     logger.info("Parsed %s observations from files in %s", str(len(obs_df)), obsdir)
-
-    return pd.merge(sim_df, obs_df, how="left", on=["well", "order", "report_step"])
+    if "report_step" in sim_df.columns:
+        return pd.merge(sim_df, obs_df, how="left", on=["well", "order", "report_step"])
+    else:
+        # Ensure backward compatibility where gendata_rft
+        return pd.merge(sim_df, obs_df, how="left", on=["well", "order"])
 
 
 def main() -> None: