diff --git a/Lib/test/test_float.py b/Lib/test/test_float.py
--- a/Lib/test/test_float.py
+++ b/Lib/test/test_float.py
@@ -576,6 +576,12 @@
         self.assertEqual(format(INF, 'f'), 'inf')
         self.assertEqual(format(INF, 'F'), 'INF')
 
+        # issue 17247
+        self.assertRaises(ValueError, format, 1.2, "<06")
+        self.assertRaises(ValueError, format, 1.2, "=06")
+        self.assertRaises(ValueError, format, 1.2, "x<06")
+        self.assertRaises(ValueError, format, 1.2, "x=06")
+
     @support.requires_IEEE_754
     def test_format_testfile(self):
         with open(format_testfile) as testfile:
diff --git a/Lib/test/test_int.py b/Lib/test/test_int.py
--- a/Lib/test/test_int.py
+++ b/Lib/test/test_int.py
@@ -410,6 +410,13 @@
             else:
                 self.fail("Expected int(%r) to raise a ValueError", s)
 
+    def test_format(self):
+        # issue 17247
+        self.assertRaises(ValueError, format, 1, "<06")
+        self.assertRaises(ValueError, format, 1, "=06")
+        self.assertRaises(ValueError, format, 1, "x<06")
+        self.assertRaises(ValueError, format, 1, "x=06")
+
 def test_main():
     support.run_unittest(IntTestCases)
 
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -194,11 +194,19 @@
     }
 
     /* The special case for 0-padding (backwards compat) */
-    if (format->fill_char == '\0' && end-pos >= 1 && READ_spec(pos) == '0') {
+    if (end-pos >= 1 && READ_spec(pos) == '0') {
+        if (format->fill_char != '\0') {
+            PyErr_Format(PyExc_ValueError,
+                "Fill character conflicts with '0' in format specifier");
+            return 0;
+        }
+        if (align_specified) {
+            PyErr_Format(PyExc_ValueError,
+                "Alignment conflicts with '0' in format specifier");
+            return 0;
+        }
         format->fill_char = '0';
-        if (!align_specified) {
-            format->align = '=';
-        }
+        format->align = '=';
         ++pos;
     }