diff --git a/nemo/collections/common/prompts/canary2.py b/nemo/collections/common/prompts/canary2.py
index c6ba2e2bc847..bef6dcfadc71 100644
--- a/nemo/collections/common/prompts/canary2.py
+++ b/nemo/collections/common/prompts/canary2.py
@@ -216,7 +216,7 @@ def canary2(cut: Cut, prompt: Canary2PromptFormatter) -> dict[str, torch.Tensor]
         eos = prompt.tokenizer.eos
     else:  # SPE
         eos = prompt.tokenizer.token_to_id(CANARY_EOS)
-    assert eos > -1, "Invalid tokenizer: tokenizer.token_to_id('{CANARY_EOS}') returned {eos}"
+    assert eos > -1, f"Invalid tokenizer: tokenizer.token_to_id('{CANARY_EOS}') returned {eos}"
     assert (
         ans["answer_ids"][-1].item() == eos
     ), f"Expected the last token in answer_ids to be EOS, but we got {ans['answer_ids']}"
diff --git a/tests/collections/common/test_canary2_prompt.py b/tests/collections/common/test_canary2_prompt.py
new file mode 100644
index 000000000000..ae3f789b95ac
--- /dev/null
+++ b/tests/collections/common/test_canary2_prompt.py
@@ -0,0 +1,39 @@
+# Copyright (c) 2025, NVIDIA CORPORATION.  All rights reserved.
+#
+# 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.
+
+"""
+Tests for Canary 2.0 prompt formatting.
+"""
+
+from unittest.mock import MagicMock
+
+import pytest
+from lhotse.testing.dummies import dummy_cut
+
+from nemo.collections.common.prompts.canary2 import canary2
+
+
+def test_canary2_assert_message_includes_eos_id():
+    """When the tokenizer returns -1 for CANARY_EOS, the assertion should report the actual id."""
+    from lhotse import SupervisionSegment
+
+    cut = dummy_cut(0, duration=1.0, supervisions=[SupervisionSegment("", "", 0, 1.0, text="hello")])
+    cut.custom = {"source_lang": "en", "target_lang": "en"}
+
+    prompt = MagicMock()
+    prompt.encode_dialog.return_value = {"answer_ids": MagicMock()}
+    prompt.tokenizer.token_to_id.return_value = -1
+
+    with pytest.raises(AssertionError, match="token_to_id.*returned -1"):
+        canary2(cut, prompt)