diff --git a/phpseclib/Math/PrimeField.php b/phpseclib/Math/PrimeField.php
index 58ea1181..289cfcbb 100644
--- a/phpseclib/Math/PrimeField.php
+++ b/phpseclib/Math/PrimeField.php
@@ -46,9 +46,9 @@ class PrimeField extends FiniteField
      */
     public function __construct(BigInteger $modulo)
     {
-        //if (!$modulo->isPrime()) {
-        //    throw new \phpseclib3\Exception\UnexpectedValueException('PrimeField requires a prime number be passed to the constructor');
-        //}
+        if (!$modulo->isPrime()) {
+            throw new \phpseclib3\Exception\UnexpectedValueException('PrimeField requires a prime number be passed to the constructor');
+        }
 
         $this->instanceID = self::$instanceCounter++;
         Integer::setModulo($this->instanceID, $modulo);
diff --git a/phpseclib/Math/PrimeField/Integer.php b/phpseclib/Math/PrimeField/Integer.php
index dee3a8e3..c24f4e18 100644
--- a/phpseclib/Math/PrimeField/Integer.php
+++ b/phpseclib/Math/PrimeField/Integer.php
@@ -249,13 +249,13 @@ class Integer extends Base
         $r = $this->value->powMod($temp, static::$modulo[$this->instanceID]);
 
         while (!$t->equals($one)) {
-            $i = clone $one;
-
-            while (!$t->powMod($two->pow($i), static::$modulo[$this->instanceID])->equals($one)) {
-                $i = $i->add($one);
+            for ($i == clone $one; $i->compare($m) < 0; $i = $i->add($one)) {
+                if ($t->powMod($two->pow($i), static::$modulo[$this->instanceID])->equals($one)) {
+                    break;
+                }
             }
 
-            if ($i->compare($m) >= 0) {
+            if ($i->compare($m) == 0) {
                 return false;
             }
             $b = $c->powMod($two->pow($m->subtract($i)->subtract($one)), static::$modulo[$this->instanceID]);