diff --git a/src/DataFrame/IO/CSV.hs b/src/DataFrame/IO/CSV.hs
index 23cca806..c4bcbd50 100644
--- a/src/DataFrame/IO/CSV.hs
+++ b/src/DataFrame/IO/CSV.hs
@@ -237,10 +237,14 @@ ghci> D.readSeparated (D.defaultReadOptions { columnSeparator = ';' }) ".\/data\
 @
 -}
 readSeparated :: ReadOptions -> FilePath -> IO DataFrame
-readSeparated !opts !path = do
-    let sep = columnSeparator opts
+readSeparated opts !path = do
     let stripUtf8Bom bs = fromMaybe bs (BL.stripPrefix "\xEF\xBB\xBF" bs)
     csvData <- stripUtf8Bom <$> BL.readFile path
+    decodeSeparated opts csvData
+
+decodeSeparated :: ReadOptions -> BL.ByteString -> IO DataFrame
+decodeSeparated !opts csvData = do
+    let sep = columnSeparator opts
     let decodeOpts = Csv.defaultDecodeOptions{Csv.decDelimiter = fromIntegral (ord sep)}
     let stream = CsvStream.decodeWith decodeOpts Csv.NoHeader csvData
 
diff --git a/src/DataFrame/Operations/Aggregation.hs b/src/DataFrame/Operations/Aggregation.hs
index 650d2159..f510a193 100644
--- a/src/DataFrame/Operations/Aggregation.hs
+++ b/src/DataFrame/Operations/Aggregation.hs
@@ -254,23 +254,25 @@ computeRowHashes indices df = runST $ do
 All ungrouped columns will be dropped.
 -}
 aggregate :: [NamedExpr] -> GroupedDataFrame -> DataFrame
-aggregate aggs gdf@(Grouped df groupingColumns valueIndices offsets) =
-    let
-        df' =
-            selectIndices
-                (VU.map (valueIndices VU.!) (VU.init offsets))
-                (select groupingColumns df)
+aggregate aggs gdf@(Grouped df groupingColumns valueIndices offsets)
+    | VU.null valueIndices = df
+    | otherwise =
+        let
+            df' =
+                selectIndices
+                    (VU.map (valueIndices VU.!) (VU.init offsets))
+                    (select groupingColumns df)
 
-        f (name, Wrap (expr :: Expr a)) d =
-            let
-                value = case interpretAggregation @a gdf expr of
-                    Left e -> throw e
-                    Right (UnAggregated _) -> throw $ UnaggregatedException (T.pack $ show expr)
-                    Right (Aggregated (TColumn col)) -> col
-             in
-                insertColumn name value d
-     in
-        fold f aggs df'
+            f (name, Wrap (expr :: Expr a)) d =
+                let
+                    value = case interpretAggregation @a gdf expr of
+                        Left e -> throw e
+                        Right (UnAggregated _) -> throw $ UnaggregatedException (T.pack $ show expr)
+                        Right (Aggregated (TColumn col)) -> col
+                in
+                    insertColumn name value d
+        in
+            fold f aggs df'
 
 selectIndices :: VU.Vector Int -> DataFrame -> DataFrame
 selectIndices xs df =
@@ -281,6 +283,8 @@ selectIndices xs df =
 
 -- | Filter out all non-unique values in a dataframe.
 distinct :: DataFrame -> DataFrame
-distinct df = selectIndices (VU.map (indices VU.!) (VU.init os)) df
+distinct df
+    | nRows df == 0 = df
+    | otherwise = selectIndices (VU.map (indices VU.!) (VU.init os)) df
   where
     (Grouped _ _ indices os) = groupBy (columnNames df) df