diff --git a/slugify/slugify.py b/slugify/slugify.py
index 9b5f27f..54843b6 100644
--- a/slugify/slugify.py
+++ b/slugify/slugify.py
@@ -49,7 +49,8 @@ def smart_truncate(
     if len(string) < max_length:
         return string
 
-    if not word_boundary:
+    # Empty separator cannot split words; fall back to hard truncate.
+    if not word_boundary or not separator:
         return string[:max_length].strip(separator)
 
     if separator not in string:
diff --git a/test.py b/test.py
index fcec4b6..743a6df 100644
--- a/test.py
+++ b/test.py
@@ -540,6 +540,17 @@ def test_smart_truncate_no_seperator(self):
         r = smart_truncate(txt, max_length=100, separator='_')
         self.assertEqual(r, txt)
 
+    def test_smart_truncate_empty_separator_word_boundary(self):
+        txt = 'helloworld'
+        self.assertEqual(
+            smart_truncate(txt, max_length=5, word_boundary=True, separator=''),
+            'hello',
+        )
+        self.assertEqual(
+            slugify('hello world', max_length=5, word_boundary=True, separator=''),
+            'hello',
+        )
+
 
 PY3 = sys.version_info.major == 3