diff --git a/yli/regress.py b/yli/regress.py
index b7af386..b386302 100644
--- a/yli/regress.py
+++ b/yli/regress.py
@@ -29,7 +29,7 @@ import warnings
 from .bayes_factors import BayesFactor, bayesfactor_afbf
 from .config import config
 from .sig_tests import FTestResult
-from .utils import Estimate, check_nan, cols_for_formula, fmt_p, formula_factor_ref_category, parse_patsy_term
+from .utils import Estimate, check_nan, cols_for_formula, convert_pandas_nullable, fmt_p, formula_factor_ref_category, parse_patsy_term
 
 def vif(df, formula=None, *, nan_policy='warn'):
 	"""
@@ -534,10 +534,8 @@ def regress(
 	if df[dep].dtype != 'float64':
 		df[dep] = df[dep].astype('float64')
 	
-	# Convert pandas nullable types for independent variables
-	for col in df.columns:
-		if df[col].dtype == 'Int64':
-			df[col] = df[col].astype('float64')
+	# Convert pandas nullable types for independent variables as this breaks statsmodels
+	df = convert_pandas_nullable(df)
 	
 	# Fit model
 	model = model_class.from_formula(formula=dep + ' ~ ' + formula, data=df, **model_kwargs)
diff --git a/yli/sig_tests.py b/yli/sig_tests.py
index 5d63321..2a63819 100644
--- a/yli/sig_tests.py
+++ b/yli/sig_tests.py
@@ -23,7 +23,7 @@ import functools
 import warnings
 
 from .config import config
-from .utils import Estimate, as_2groups, check_nan, fmt_p
+from .utils import Estimate, as_2groups, check_nan, convert_pandas_nullable, fmt_p
 
 # ----------------
 # Student's t test
@@ -334,6 +334,9 @@ def mannwhitney(df, dep, ind, *, nan_policy='warn', brunnermunzel=True, use_cont
 	# Check for/clean NaNs
 	df = check_nan(df[[ind, dep]], nan_policy)
 	
+	# Convert pandas nullable types for independent variables as this breaks statsmodels
+	df = convert_pandas_nullable(df)
+	
 	# Ensure 2 groups for ind
 	group1, data1, group2, data2 = as_2groups(df, dep, ind)
 	
diff --git a/yli/utils.py b/yli/utils.py
index 5887505..a19acc5 100644
--- a/yli/utils.py
+++ b/yli/utils.py
@@ -41,6 +41,18 @@ def check_nan(df, nan_policy):
 	else:
 		raise Exception('Invalid nan_policy, expected "raise", "warn" or "omit"')
 
+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()
+	
+	for col in df.columns:
+		if df[col].dtype == 'Int64':
+			df[col] = df[col].astype('int')
+	
+	return df
+
 def as_2groups(df, data, group):
 	"""Group the data by the given variable, ensuring only 2 groups"""