diff --git a/src/spdx_tools/spdx/parser/tagvalue/lexer.py b/src/spdx_tools/spdx/parser/tagvalue/lexer.py
index 9a857827b..73328e9c3 100644
--- a/src/spdx_tools/spdx/parser/tagvalue/lexer.py
+++ b/src/spdx_tools/spdx/parser/tagvalue/lexer.py
@@ -139,7 +139,7 @@ def t_text_error(self, t):
 
     @TOKEN(
         r":\s*(ADLER32|BLAKE2b-256|BLAKE2b-384|BLAKE2b-512|BLAKE3|MD2|MD4|MD5|MD6|SHA1|SHA224|SHA256|SHA384|SHA512|"
-        r"SHA3-256|SHA3-384|SHA3-512):\s*([a-f0-9]*)"
+        r"SHA3-256|SHA3-384|SHA3-512):\s*([a-f0-9]+)"
     )
     def t_CHECKSUM(self, t):
         t.value = t.value[1:].strip()
diff --git a/tests/spdx/parser/tagvalue/test_file_parser.py b/tests/spdx/parser/tagvalue/test_file_parser.py
index aedf197b5..7749af66e 100644
--- a/tests/spdx/parser/tagvalue/test_file_parser.py
+++ b/tests/spdx/parser/tagvalue/test_file_parser.py
@@ -68,3 +68,17 @@ def test_parse_invalid_file():
         "Error while parsing File: ['Invalid FileType: SOUCE. Line 3', 'Error while "
         "parsing FileChecksum: Token did not match specified grammar rule. Line: 5']"
     ]
+
+
+def test_parse_file_uppercase_checksum_raises():
+    # An uppercase checksum value used to silently produce Checksum(SHA1, "")
+    # instead of raising; the lexer must reject it as a grammar error.
+    file_str = "\n".join(
+        [
+            "FileName: f",
+            "SPDXID: SPDXRef-File",
+            "FileChecksum: SHA1: D6A770BA38583ED4BB4525BD96E50461655D2758",
+        ]
+    )
+    with pytest.raises(SPDXParsingError):
+        Parser().parse("\n".join([DOCUMENT_STR, file_str]))