diff --git a/_pytest/_argcomplete.py b/_pytest/_argcomplete.py
index 1ba1ecc1e..965ec7951 100644
--- a/_pytest/_argcomplete.py
+++ b/_pytest/_argcomplete.py
@@ -78,7 +78,8 @@ class FastFilesCompleter:
         completion = []
         globbed = []
         if '*' not in prefix and '?' not in prefix:
-            if prefix[-1] == os.path.sep:  # we are on unix, otherwise no bash
+            # we are on unix, otherwise no bash
+            if not prefix or prefix[-1] == os.path.sep:
                 globbed.extend(glob(prefix + '.*'))
             prefix += '*'
         globbed.extend(glob(prefix))
@@ -98,7 +99,7 @@ if os.environ.get('_ARGCOMPLETE'):
     filescompleter = FastFilesCompleter()
 
     def try_argcomplete(parser):
-        argcomplete.autocomplete(parser)
+        argcomplete.autocomplete(parser, always_complete_options=False)
 else:
     def try_argcomplete(parser):
         pass
diff --git a/changelog/2748.bugfix b/changelog/2748.bugfix
new file mode 100644
index 000000000..b5b6f5839
--- /dev/null
+++ b/changelog/2748.bugfix
@@ -0,0 +1 @@
+Fix crash in tab completion when no prefix is given.
diff --git a/testing/test_argcomplete.py b/testing/test_argcomplete.py
index b07f6e83b..c92612577 100644
--- a/testing/test_argcomplete.py
+++ b/testing/test_argcomplete.py
@@ -82,7 +82,7 @@ class TestArgComplete(object):
         from _pytest._argcomplete import FastFilesCompleter
         ffc = FastFilesCompleter()
         fc = FilesCompleter()
-        for x in '/ /d /data qqq'.split():
+        for x in ['/', '/d', '/data', 'qqq', '']:
             assert equal_with_bash(x, ffc, fc, out=py.std.sys.stdout)
 
     @pytest.mark.skipif("sys.platform in ('win32', 'darwin')")