diff --git a/numpy/core/src/umath/dispatching.c b/numpy/core/src/umath/dispatching.c
index b8f102b3dff2..2be2d9e17149 100644
--- a/numpy/core/src/umath/dispatching.c
+++ b/numpy/core/src/umath/dispatching.c
@@ -914,8 +914,13 @@ promote_and_get_ufuncimpl(PyUFuncObject *ufunc,
             signature[i] = (PyArray_DTypeMeta *)PyTuple_GET_ITEM(all_dtypes, i);
             Py_INCREF(signature[i]);
         }
-        else {
-            assert((PyObject *)signature[i] == PyTuple_GET_ITEM(all_dtypes, i));
+        else if ((PyObject *)signature[i] != PyTuple_GET_ITEM(all_dtypes, i)) {
+            /*
+             * If signature is forced the cache may contain an incompatible
+             * loop found via promotion (signature not enforced).  Reject it.
+             */
+            raise_no_loop_found_error(ufunc, (PyObject **)op_dtypes);
+            return NULL;
         }
     }
 
diff --git a/numpy/core/tests/test_ufunc.py b/numpy/core/tests/test_ufunc.py
index 852044d32fcc..d1510c6fdbe8 100644
--- a/numpy/core/tests/test_ufunc.py
+++ b/numpy/core/tests/test_ufunc.py
@@ -492,6 +492,15 @@ def test_partial_signature_mismatch(self, casting):
         with pytest.raises(TypeError):
             np.ldexp(1., np.uint64(3), signature=(None, None, "d"))
 
+    def test_partial_signature_mismatch_with_cache(self):
+        with pytest.raises(TypeError):
+            np.add(np.float16(1), np.uint64(2), sig=("e", "d", None))
+        # Ensure e,d->None is in the dispatching cache (double loop)
+        np.add(np.float16(1), np.float64(2))
+        # The error must still be raised:
+        with pytest.raises(TypeError):
+            np.add(np.float16(1), np.uint64(2), sig=("e", "d", None))
+
     def test_use_output_signature_for_all_arguments(self):
         # Test that providing only `dtype=` or `signature=(None, None, dtype)`
         # is sufficient if falling back to a homogeneous signature works.
diff --git a/numpy/core/tests/test_umath.py b/numpy/core/tests/test_umath.py
index f12307e30ba4..4160dc9b1ffb 100644
--- a/numpy/core/tests/test_umath.py
+++ b/numpy/core/tests/test_umath.py
@@ -226,6 +226,14 @@ def __ne__(self, other):
         a = np.array([np.nan], dtype=object)
         assert_equal(np.not_equal(a, a), [True])
 
+    def test_error_in_equal_reduce(self):
+        # gh-20929
+        # make sure np.equal.reduce raises a TypeError if an array is passed
+        # without specifying the dtype
+        a = np.array([0, 0])
+        assert_equal(np.equal.reduce(a, dtype=bool), True)
+        assert_raises(TypeError, np.equal.reduce, a)
+
 
 class TestAdd:
     def test_reduce_alignment(self):