diff --git a/vunit/test/acceptance/test_ui.py b/vunit/test/acceptance/test_ui.py
index 71fd0f361..fa7ea2429 100644
--- a/vunit/test/acceptance/test_ui.py
+++ b/vunit/test/acceptance/test_ui.py
@@ -16,7 +16,7 @@
 from os import remove
 import re
 from re import MULTILINE
-from vunit.ui import VUnit
+from vunit.ui import VUnit, VHDL_EXTENSIONS, VERILOG_EXTENSIONS
 
 
 class TestPreprocessor(object):
@@ -86,7 +86,7 @@ def _create_ui(self):
         ui.add_library('lib')
         return ui
 
-    def _create_temp_files(self, num_files):
+    def _create_temp_files(self, num_files, file_suffix='.vhd'):
         """
         Create and return num_files temporary files containing the
         same source code but with different entity names depending on
@@ -94,7 +94,7 @@ def _create_temp_files(self, num_files):
         """
         files = [None] * num_files
         for i in range(num_files):
-            with NamedTemporaryFile(mode='w', suffix='.vhd', delete=False) as files[i]:
+            with NamedTemporaryFile(mode='w', suffix=file_suffix, delete=False) as files[i]:
                 files[i].write(self._source.substitute(entity='foo%d' % i))
         return files
 
@@ -196,6 +196,27 @@ def test_locally_specified_preprocessors_should_be_used_instead_of_any_globally_
 
         self._delete_temp_files(files)
 
+    def test_supported_source_file_suffixes(self):
+        """Test adding a supported filetype, of any case, is accepted."""
+        ui = self._create_ui()
+        accepted_extensions = VHDL_EXTENSIONS + VERILOG_EXTENSIONS
+        allowable_extensions = [ext for ext in accepted_extensions]
+        allowable_extensions.extend([ext.upper() for ext in accepted_extensions])
+        allowable_extensions.append(VHDL_EXTENSIONS[0][0] + VHDL_EXTENSIONS[0][1].upper() +
+                                    VHDL_EXTENSIONS[0][2:])  # mixed case
+        for ext in allowable_extensions:
+            files = self._create_temp_files(1, ext)
+            ui.add_source_files(files[0].name, 'lib')
+            self._delete_temp_files(files)
+
+    def test_unsupported_source_file_suffixes(self):
+        """Test adding an unsupported filetype is rejected"""
+        ui = self._create_ui()
+        bogus_ext = '.docx'  # obviously not an HDL file
+        files = self._create_temp_files(1, bogus_ext)
+        self.assertRaises(RuntimeError, ui.add_source_files, files[0].name, 'lib')
+        self._delete_temp_files(files)
+
 
 class TestUiEncoding(unittest.TestCase):
     """
diff --git a/vunit/ui.py b/vunit/ui.py
index a025761f3..3a9a07870 100644
--- a/vunit/ui.py
+++ b/vunit/ui.py
@@ -43,6 +43,10 @@
 import logging
 LOGGER = logging.getLogger(__name__)
 
+# lower case representation of supported extensions
+VHDL_EXTENSIONS = (".vhd", ".vhdl")
+VERILOG_EXTENSIONS = (".v", ".vp", ".sv")
+
 
 class VUnit(object):  # pylint: disable=too-many-instance-attributes, too-many-public-methods
     """
@@ -665,9 +669,9 @@ def file_type_of(file_name):
     Return the file type of file_name based on the file ending
     """
     _, ext = splitext(file_name)
-    if ext in (".vhd", ".vhdl"):
+    if ext.lower() in VHDL_EXTENSIONS:
         return "vhdl"
-    elif ext in (".v", ".vp", ".sv"):
+    elif ext.lower() in VERILOG_EXTENSIONS:
         return "verilog"
     else:
         raise RuntimeError("Unknown file ending '%s' of %s" % (ext, file_name))