diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
index 6f234f667d11..340eb8db7ed9 100644
--- a/lib/matplotlib/cbook/__init__.py
+++ b/lib/matplotlib/cbook/__init__.py
@@ -412,7 +412,12 @@ def _strip_comment(s):
         elif 0 <= hash_pos < quote_pos:
             return s[:hash_pos].strip()
         else:
-            pos = s.find('"', quote_pos + 1) + 1  # behind closing quote
+            closing_quote_pos = s.find('"', quote_pos + 1)
+            if closing_quote_pos < 0:
+                raise ValueError(
+                    f"Missing closing quote in: {s!r}. If you need a double-"
+                    'quote inside a string, use escaping: e.g. "the \" char"')
+            pos = closing_quote_pos + 1  # behind closing quote
 
 
 def is_writable_file_like(obj):
diff --git a/lib/matplotlib/tests/test_cbook.py b/lib/matplotlib/tests/test_cbook.py
index 33ed3c2beaf1..aacc0935a2e3 100644
--- a/lib/matplotlib/tests/test_cbook.py
+++ b/lib/matplotlib/tests/test_cbook.py
@@ -424,6 +424,11 @@ def test_strip_comment(line, result):
     assert cbook._strip_comment(line) == result
 
 
+def test_strip_comment_invalid():
+    with pytest.raises(ValueError, match="Missing closing quote"):
+        cbook._strip_comment('grid.color: "aa')
+
+
 def test_sanitize_sequence():
     d = {'a': 1, 'b': 2, 'c': 3}
     k = ['a', 'b', 'c']