diff --git a/erddapy/core/interfaces.py b/erddapy/core/interfaces.py
index 7f451a56..59c3947c 100644
--- a/erddapy/core/interfaces.py
+++ b/erddapy/core/interfaces.py
@@ -17,7 +17,9 @@
 
 
 def to_pandas(
-    url: str, requests_kwargs: Optional[Dict] = None, **pandas_kwargs
+    url: str,
+    requests_kwargs: Optional[Dict] = None,
+    pandas_kwargs: Optional[Dict] = None,
 ) -> pd.DataFrame:
     """
     Convert a URL to Pandas DataFrame.
@@ -28,6 +30,8 @@ def to_pandas(
     """
     if requests_kwargs is None:
         requests_kwargs = {}
+    if pandas_kwargs is None:
+        pandas_kwargs = {}
     data = urlopen(url, **requests_kwargs)
     try:
         return pd.read_csv(data, **pandas_kwargs)
@@ -60,18 +64,20 @@ def to_xarray(
     url: str,
     response="opendap",
     requests_kwargs: Optional[Dict] = None,
-    **xarray_kwargs,
+    xarray_kwargs: Optional[Dict] = None,
 ) -> xr.Dataset:
     """
     Convert a URL to an xarray dataset.
 
     url: URL to request data from.
     response: type of response to be requested from the server.
-    requests_kwargs: arguments to be passed to urlopen method
+    requests_kwargs: arguments to be passed to urlopen method.
     xarray_kwargs: kwargs to be passed to third-party library (xarray).
     """
     if requests_kwargs is None:
         requests_kwargs = {}
+    if xarray_kwargs is None:
+        xarray_kwargs = {}
     if response == "opendap":
         return xr.open_dataset(url, **xarray_kwargs)
     else:
@@ -79,7 +85,11 @@ def to_xarray(
         return xr.open_dataset(xr.backends.NetCDF4DataStore(nc), **xarray_kwargs)
 
 
-def to_iris(url: str, requests_kwargs: Optional[Dict] = None, **iris_kwargs):
+def to_iris(
+    url: str,
+    requests_kwargs: Optional[Dict] = None,
+    iris_kwargs: Optional[Dict] = None,
+):
     """
     Convert a URL to an iris CubeList.
 
@@ -89,6 +99,8 @@ def to_iris(url: str, requests_kwargs: Optional[Dict] = None, **iris_kwargs):
     """
     if requests_kwargs is None:
         requests_kwargs = {}
+    if iris_kwargs is None:
+        iris_kwargs = {}
     data = urlopen(url, **requests_kwargs)
     with _tempnc(data) as tmp:
         cubes = iris.load_raw(tmp, **iris_kwargs)