diff --git a/pytplot/importers/cdf_to_tplot.py b/pytplot/importers/cdf_to_tplot.py
index 38b71b5..4d7b1ee 100644
--- a/pytplot/importers/cdf_to_tplot.py
+++ b/pytplot/importers/cdf_to_tplot.py
@@ -95,6 +95,12 @@ def cdf_to_tplot(filenames, mastercdf=None, varformat=None, exclude_format=None,
         new_cdflib = False
         logging.debug("Using old version of cdflib (%s)", cdflib.__version__)
 
+    if prefix is None:
+        prefix = ''
+
+    if suffix is None:
+        suffix = ''
+
     if not isinstance(varnames, list):
         varnames = [varnames]
 
diff --git a/pytplot/importers/netcdf_to_tplot.py b/pytplot/importers/netcdf_to_tplot.py
index 6e22923..f789f9e 100644
--- a/pytplot/importers/netcdf_to_tplot.py
+++ b/pytplot/importers/netcdf_to_tplot.py
@@ -86,6 +86,11 @@ def netcdf_to_tplot(
 
     stored_variables = []
 
+    if prefix is None:
+        prefix = ''
+    if suffix is None:
+        suffix = ''
+
     if isinstance(filenames, str):
         filenames = [filenames]
     elif isinstance(filenames, list):
diff --git a/pytplot/importers/sts_to_tplot.py b/pytplot/importers/sts_to_tplot.py
index 00ebf2a..f0de86f 100644
--- a/pytplot/importers/sts_to_tplot.py
+++ b/pytplot/importers/sts_to_tplot.py
@@ -40,6 +40,11 @@ def sts_to_tplot(sts_file=None, read_only=False, prefix='', suffix='', merge=Tru
     sts_dict = {}
     stored_variables = []
 
+    if prefix is None:
+        prefix = ''
+    if suffix is None:
+        suffix = ''
+
     # Code assumes a list of STS files
     if isinstance(sts_file, str):
         sts_file = [sts_file]