diff --git a/wolfcrypt/src/port/maxim/max3266x.c b/wolfcrypt/src/port/maxim/max3266x.c
index d48e1efe9ad..7d0dcaca845 100644
--- a/wolfcrypt/src/port/maxim/max3266x.c
+++ b/wolfcrypt/src/port/maxim/max3266x.c
@@ -2495,6 +2495,10 @@ int hw_mulmod(mp_int* multiplier, mp_int* multiplicand, mp_int* mod,
         return MP_VAL;
     }
     if ((multiplier->used == 0) || (multiplicand->used == 0)) {
+        /* A zero modulus is invalid whatever the operands are. */
+        if (mod->used == 0) {
+            return MP_VAL;
+        }
         mp_zero(result);
         return 0;
     }
@@ -2591,6 +2595,10 @@ int hw_sqrmod(mp_int* base, mp_int* mod, mp_int* result)
         return MP_VAL;
     }
     if (base->used == 0) {
+        /* A zero modulus is invalid whatever the base is. */
+        if (mod->used == 0) {
+            return MP_VAL;
+        }
         mp_zero(result);
         return 0;
     }
diff --git a/wolfcrypt/src/sp_int.c b/wolfcrypt/src/sp_int.c
index 83017ae29f5..3340d0bc672 100644
--- a/wolfcrypt/src/sp_int.c
+++ b/wolfcrypt/src/sp_int.c
@@ -12292,6 +12292,7 @@ int sp_mul(const sp_int* a, const sp_int* b, sp_int* r)
  * @param [out] r  SP integer result.
  *
  * @return  MP_OKAY on success.
+ * @return  MP_VAL when m is 0.
  * @return  MP_MEM when dynamic memory allocation fails.
  */
 static int _sp_mulmod_tmp(const sp_int* a, const sp_int* b, const sp_int* m,
@@ -12300,7 +12301,15 @@ static int _sp_mulmod_tmp(const sp_int* a, const sp_int* b, const sp_int* m,
     int err = MP_OKAY;
 
     if (sp_iszero(a) || sp_iszero(b)) {
-        _sp_zero(r);
+        /* Only reached from sp_mulmod() when the result aliases the modulus.
+         * The zero-operand short-circuit would otherwise bypass the sp_mod()
+         * validation that the non-zero operand path relies on. */
+        if (sp_iszero(m)) {
+            err = MP_VAL;
+        }
+        else {
+            _sp_zero(r);
+        }
     }
     else {
         /* Create temporary for multiplication result. */
@@ -12334,6 +12343,7 @@ static int _sp_mulmod_tmp(const sp_int* a, const sp_int* b, const sp_int* m,
  * @param [out] r  SP integer result.
  *
  * @return  MP_OKAY on success.
+ * @return  MP_VAL when m is 0.
  * @return  MP_MEM when dynamic memory allocation fails.
  */
 static int _sp_mulmod(const sp_int* a, const sp_int* b, const sp_int* m,
@@ -17501,6 +17511,7 @@ int sp_sqr(const sp_int* a, sp_int* r)
  * @param [out] r  SP integer result.
  *
  * @return  MP_OKAY on success.
+ * @return  MP_VAL when m is 0.
  * @return  MP_MEM when dynamic memory allocation fails.
  */
 static int _sp_sqrmod(const sp_int* a, const sp_int* m, sp_int* r)
@@ -17508,7 +17519,15 @@ static int _sp_sqrmod(const sp_int* a, const sp_int* m, sp_int* r)
     int err = MP_OKAY;
 
     if (sp_iszero(a)) {
-        _sp_zero(r);
+        /* Only reached from sp_sqrmod() when the result aliases the modulus.
+         * The zero-operand short-circuit would otherwise bypass the sp_mod()
+         * validation that the non-zero operand path relies on. */
+        if (sp_iszero(m)) {
+            err = MP_VAL;
+        }
+        else {
+            _sp_zero(r);
+        }
     }
     else {
         /* Create temporary for multiplication result. */
diff --git a/wolfcrypt/test/test.c b/wolfcrypt/test/test.c
index 2953550ae90..5dcda911aa8 100644
--- a/wolfcrypt/test/test.c
+++ b/wolfcrypt/test/test.c
@@ -75903,6 +75903,39 @@ static wc_test_ret_t mp_test_mulmod_sqrmod(mp_int* a, mp_int* b, mp_int* m,
     if (!mp_iszero(m))
         return WC_TEST_RET_ENC_NC;
 
+    /* A zero modulus is invalid whether or not the result aliases it. The
+     * zero-operand short-circuits must not hide it. */
+    ret = mp_set(a, 0);
+    if (ret == MP_OKAY)
+        ret = mp_set(b, 0x5);
+    if (ret == MP_OKAY)
+        ret = mp_set(m, 0);
+    if (ret != MP_OKAY)
+        return WC_TEST_RET_ENC_EC(ret);
+    if (mp_sqrmod(a, m, r) != WC_NO_ERR_TRACE(MP_VAL))
+        return WC_TEST_RET_ENC_NC;
+    if (mp_sqrmod(a, m, m) != WC_NO_ERR_TRACE(MP_VAL))
+        return WC_TEST_RET_ENC_NC;
+    ret = mp_set(m, 0);
+    if (ret != MP_OKAY)
+        return WC_TEST_RET_ENC_EC(ret);
+    if (mp_mulmod(a, b, m, r) != WC_NO_ERR_TRACE(MP_VAL))
+        return WC_TEST_RET_ENC_NC;
+    if (mp_mulmod(a, b, m, m) != WC_NO_ERR_TRACE(MP_VAL))
+        return WC_TEST_RET_ENC_NC;
+    /* Cover the other half of the zero-operand test in _sp_mulmod_tmp. */
+    ret = mp_set(a, 0x5);
+    if (ret == MP_OKAY)
+        ret = mp_set(b, 0);
+    if (ret == MP_OKAY)
+        ret = mp_set(m, 0);
+    if (ret != MP_OKAY)
+        return WC_TEST_RET_ENC_EC(ret);
+    if (mp_mulmod(a, b, m, r) != WC_NO_ERR_TRACE(MP_VAL))
+        return WC_TEST_RET_ENC_NC;
+    if (mp_mulmod(a, b, m, m) != WC_NO_ERR_TRACE(MP_VAL))
+        return WC_TEST_RET_ENC_NC;
+
     return 0;
 }