diff --git a/yli/utils.py b/yli/utils.py
index ce95f0a..9bb0134 100644
--- a/yli/utils.py
+++ b/yli/utils.py
@@ -45,14 +45,18 @@ def check_nan(df, nan_policy):
 def convert_pandas_nullable(df):
 	"""Convert pandas nullable dtypes (e.g. Int64) to non-nullable numpy dtypes"""
 	
-	# TODO: Can we avoid this copy?
-	df = df.copy()
+	# Avoid copy if possible
+	df_cleaned = None
 	
 	for col in df.columns:
-		if df[col].dtype == 'Int64':
-			df[col] = df[col].astype('int')
+		if df[col].dtype in ('Int64', 'Float64'):
+			if df_cleaned is None:
+				df_cleaned = df.copy()
+			df_cleaned[col] = df[col].astype(str(df[col].dtype).lower())
 	
-	return df
+	if df_cleaned is None:
+		return df
+	return df_cleaned
 
 def as_2groups(df, data, group):
 	"""Group the data by the given variable, ensuring only 2 groups"""