diff --git a/tests/utils/reward_score/test_gsm8k_on_cpu.py b/tests/utils/reward_score/test_gsm8k_on_cpu.py
new file mode 100644
index 00000000000..a60530d9fe2
--- /dev/null
+++ b/tests/utils/reward_score/test_gsm8k_on_cpu.py
@@ -0,0 +1,40 @@
+# Copyright 2024 Bytedance Ltd. and/or its affiliates
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+
+from verl.utils.reward_score import gsm8k
+
+
+@pytest.mark.parametrize(
+    "solution_str, expected",
+    [
+        ("The answer is 42", "42"),
+        ("first 7 then 13", "13"),
+        ("negative -5 here", "-5"),
+        # Only invalid tokens (a stray period): there is no real number, so the
+        # extraction must return None rather than the invalid "." token.
+        ("The result is just a period .", None),
+        ("no digits at all", None),
+    ],
+)
+def test_extract_solution_flexible(solution_str, expected):
+    assert gsm8k.extract_solution(solution_str, method="flexible") == expected
+
+
+def test_flexible_no_valid_number_scores_zero():
+    # A stray "." used to be extracted as the answer, so a non-zero format_score
+    # leaked to outputs that contain no real number. With no valid number the
+    # answer is None and the score is 0.
+    assert gsm8k.compute_score("just a period .", "42", method="flexible", format_score=0.1) == 0
diff --git a/verl/utils/reward_score/gsm8k.py b/verl/utils/reward_score/gsm8k.py
index 98a8c24dc8c..f86165560a6 100644
--- a/verl/utils/reward_score/gsm8k.py
+++ b/verl/utils/reward_score/gsm8k.py
@@ -43,8 +43,9 @@ def extract_solution(solution_str, method="strict"):
         else:
             invalid_str = ["", "."]
             # find the last number that is not '.'
-            for final_answer in reversed(answer):
-                if final_answer not in invalid_str:
+            for candidate in reversed(answer):
+                if candidate not in invalid_str:
+                    final_answer = candidate
                     break
     return final_answer