diff --git a/Lib/test/test_complex.py b/Lib/test/test_complex.py
index cee49343e268a2..1980fc4092644e 100644
--- a/Lib/test/test_complex.py
+++ b/Lib/test/test_complex.py
@@ -345,6 +345,9 @@ def split_zeros(x):
         self.assertEqual(type(complex("1"*500)), complex)
         # check whitespace processing
         self.assertEqual(complex('\N{EM SPACE}(\N{EN SPACE}1+1j ) '), 1+1j)
+        # Invalid unicode string
+        # See bpo-34087
+        self.assertRaises(ValueError, complex, '\u3053\u3093\u306b\u3061\u306f')
 
         class EvilExc(Exception):
             pass
diff --git a/Lib/test/test_float.py b/Lib/test/test_float.py
index c5ca50c8f71206..61551e5c99031b 100644
--- a/Lib/test/test_float.py
+++ b/Lib/test/test_float.py
@@ -60,6 +60,9 @@ def test_float(self):
         # extra long strings should not be a problem
         float(b'.' + b'1'*1000)
         float('.' + '1'*1000)
+        # Invalid unicode string
+        # See bpo-34087
+        self.assertRaises(ValueError, float, '\u3053\u3093\u306b\u3061\u306f')
 
     def test_underscores(self):
         for lit in VALID_UNDERSCORE_LITERALS:
diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py
index fd15f04aceca8f..140ace18dd1e79 100644
--- a/Lib/test/test_long.py
+++ b/Lib/test/test_long.py
@@ -373,6 +373,10 @@ def test_long(self):
         for base in invalid_bases:
             self.assertRaises(ValueError, int, '42', base)
 
+        # Invalid unicode string
+        # See bpo-34087
+        self.assertRaises(ValueError, int, '\u3053\u3093\u306b\u3061\u306f')
+
 
     def test_conversion(self):
 
diff --git a/Python/pystrtod.c b/Python/pystrtod.c
index 64d0c52e487958..58278c24be9c28 100644
--- a/Python/pystrtod.c
+++ b/Python/pystrtod.c
@@ -391,6 +391,8 @@ _Py_string_to_number_with_underscores(
     char *dup, *end;
     PyObject *result;
 
+    assert(s[orig_len] == '\0');
+
     if (strchr(s, '_') == NULL) {
         return innerfunc(s, orig_len, arg);
     }