diff --git a/mp_prime_frobenius_underwood.c b/mp_prime_frobenius_underwood.c
index ced0583..0e2ba64 100644
--- a/mp_prime_frobenius_underwood.c
+++ b/mp_prime_frobenius_underwood.c
@@ -39,12 +39,8 @@ mp_err mp_prime_frobenius_underwood(const mp_int *N, mp_bool *result)
           (a==14) || (a==18) || (a==23) || (a==26) || (a==28)) {
          continue;
       }
-      /* (32764^2 - 4) < 2^31, no bigint for >MP_8BIT needed) */
-      mp_set_u32(&T1z, (uint32_t)a);
 
-      if ((err = mp_sqr(&T1z, &T1z)) != MP_OKAY)                  goto LBL_FU_ERR;
-
-      if ((err = mp_sub_d(&T1z, 4uL, &T1z)) != MP_OKAY)           goto LBL_FU_ERR;
+      mp_set_i32(&T1z, (int32_t)((a * a) - 4));
 
       if ((err = mp_kronecker(&T1z, N, &j)) != MP_OKAY)           goto LBL_FU_ERR;
 
diff --git a/tommath_class.h b/tommath_class.h
index c6dcde1..52c5e45 100644
--- a/tommath_class.h
+++ b/tommath_class.h
@@ -659,10 +659,9 @@
 #   define MP_MUL_C
 #   define MP_MUL_D_C
 #   define MP_SET_C
+#   define MP_SET_I32_C
 #   define MP_SET_U32_C
-#   define MP_SQR_C
 #   define MP_SUB_C
-#   define MP_SUB_D_C
 #   define S_MP_GET_BIT_C
 #endif