doc/news.rst           | 2 ++
 pyderasn.py            | 2 ++
 tests/test_pyderasn.py | 6 ++++++

diff --git a/doc/news.rst b/doc/news.rst
index 2c42c2e4ebc7cd556a03f0eb82f684e3987b4167850fb25ce2e444a292256979..d8d9dc32b875d3ce15352117fb0da6c14c272ae08988cccfd27b78a46b5926d1 100644
--- a/doc/news.rst
+++ b/doc/news.rst
@@ -7,6 +7,8 @@ 7.7
 ---
 * Strictly check that tag's long encoded form does not contain leading zero
   (X.690 8.1.2.4.2 (c))
+* Strictly check that tag's long form is used in expected way for small values
+  (X.690 8.1.2.2)
 
 .. _release7.6:
 
diff --git a/pyderasn.py b/pyderasn.py
index b4770043c5e918b1fc83f82e48b9bd255ee8d31e402414a351c0fdfa8d1bc96d..c402727b0f3fcd5855eff16419fb671524d4aa1811adfa35a9ea8beed767b6a2 100755
--- a/pyderasn.py
+++ b/pyderasn.py
@@ -1558,6 +1558,8 @@         if i == len(data):
             raise DecodeError("unfinished tag")
         if indexbytes(data, i) & 0x80 == 0:
             break
+    if i == 1 and indexbytes(data, 1) < 0x1F:
+        raise DecodeError("unexpected long form")
     if i > 1 and indexbytes(data, 1) & 0x7F == 0:
         raise DecodeError("leading zero byte in tag value")
     i += 1
diff --git a/tests/test_pyderasn.py b/tests/test_pyderasn.py
index 387bd6758f4a935786cbaa5ad3b6d10f45a1222d0570ab1efb1c7fc3ff598be8..dcccc7a2a7e1a54298f146474a658ce0726e6e279e971ca06c01736075f5e6c0 100644
--- a/tests/test_pyderasn.py
+++ b/tests/test_pyderasn.py
@@ -283,6 +283,12 @@         raw = b"".join((raw[:1], b"\x80", raw[1:]))
         with assertRaisesRegex(self, DecodeError, "leading zero byte"):
             tag_strip(raw)
 
+    @given(tag_classes, tag_forms, integers(max_value=30, min_value=0))
+    def test_unexpected_long_form(self, klass, form, num):
+        raw = int2byte(klass | form | 31) + int2byte(num)
+        with assertRaisesRegex(self, DecodeError, "unexpected long form"):
+            tag_strip(raw)
+
 
 class TestLenCoder(TestCase):
     @settings(max_examples=LONG_TEST_MAX_EXAMPLES)