diff --git a/massql/msql_engine_filters.py b/massql/msql_engine_filters.py
index 127034c..7dbe0c5 100644
--- a/massql/msql_engine_filters.py
+++ b/massql/msql_engine_filters.py
@@ -195,6 +195,10 @@ def _merge_filter_cardinality(condition, ms_df_list):
             # Figuring out the scans
             ms_peak_df = pd.concat(ms_df_list)
 
+            if "mzenumeration" not in ms_peak_df.columns:
+                # MS1 data does not have per-peak enumeration — cardinality filtering is not applicable
+                return ms_peak_df
+
             enumeration_df = ms_peak_df.groupby(["scan", "mzenumeration"]).first().reset_index()
             enumeration_df = enumeration_df.groupby(["scan"]).count()
             enumeration_df = enumeration_df[enumeration_df["mzenumeration"] >= min_cardinality]