diff --git a/askama_parser/src/expr.rs b/askama_parser/src/expr.rs
index 3ebc4ba3..50e719b2 100644
--- a/askama_parser/src/expr.rs
+++ b/askama_parser/src/expr.rs
@@ -1029,7 +1029,7 @@ impl<'a: 'l, 'l> Suffix<'a> {
                 // raw string string or identifier
                 "r" => None,
                 // a simple identifier
-                _ if hashes == 0 => return Ok(()),
+                _ if hashes == 0 && quot.is_none() => return Ok(()),
                 // reserved prefix: reject
                 _ => {
                     return cut_error!(
diff --git a/askama_parser/src/tests.rs b/askama_parser/src/tests.rs
index e6facca0..1a42a125 100644
--- a/askama_parser/src/tests.rs
+++ b/askama_parser/src/tests.rs
@@ -1710,3 +1710,17 @@ fn regression_tests_span_change() {
         ))]
     );
 }
+
+#[test]
+fn test_unclosed_prefixed_string() {
+    // Regression test for <https://issues.oss-fuzz.com/issues/440177293>.
+    let syntax = Syntax::default();
+    for test in ["{{ x!(i\") }}", "{{ x!(i\"\") }}"] {
+        assert!(
+            Ast::from_str(test, None, &syntax)
+                .unwrap_err()
+                .to_string()
+                .contains("reserved prefix `i#`")
+        );
+    }
+}
diff --git a/fuzzing/fuzz/artifacts/derive/clusterfuzz-testcase-minimized-derive-4654117948817408 b/fuzzing/fuzz/artifacts/derive/clusterfuzz-testcase-minimized-derive-4654117948817408
new file mode 100644
index 00000000..90707d42
Binary files /dev/null and b/fuzzing/fuzz/artifacts/derive/clusterfuzz-testcase-minimized-derive-4654117948817408 differ