diff --git a/lib/MathParser/Expressions/Number.php b/lib/MathParser/Expressions/Number.php
index 6bd2bbe..7b49ecc 100644
--- a/lib/MathParser/Expressions/Number.php
+++ b/lib/MathParser/Expressions/Number.php
@@ -8,7 +8,7 @@ class Number extends Expression
 {
     public function __construct($value)
     {
-        parent::__construct(+$value);
+        parent::__construct($value);
     }
     
     public function operate(array &$stack)
diff --git a/lib/MathParser/Math.php b/lib/MathParser/Math.php
index 08b31ae..a353f00 100644
--- a/lib/MathParser/Math.php
+++ b/lib/MathParser/Math.php
@@ -77,7 +77,7 @@ public function parse($string)
     
     public function registerVariable($name, $value)
     {
-        $this->assertVariableIsNumber($value);
+        $this->assertVariableIsNumberOrNull($value);
     
         $this->variables[$name] = $value;
     }
@@ -198,7 +198,7 @@ public function clearVariables()
      */
     public function setVariables(array $variables)
     {
-        $this->assertVariablesAreNumbers($variables);
+        $this->assertVariablesAreNumbersOrNull($variables);
         $this->variables = $variables;
     }
     
@@ -211,17 +211,17 @@ private function substituteVariables(array &$stack, array $variables)
         }
     }
     
-    private function assertVariablesAreNumbers(array $variables)
+    private function assertVariablesAreNumbersOrNull(array $variables)
     {
         foreach ($variables as $variable) {
-            $this->assertVariableIsNumber($variable);
+            $this->assertVariableIsNumberOrNull($variable);
         }
     }
     
-    private function assertVariableIsNumber($variable)
+    private function assertVariableIsNumberOrNull($variable)
     {
-        if (!is_int($variable) && !is_float($variable)) {
-            throw new \InvalidArgumentException('provided variable is not a number');
+        if (!is_int($variable) && !is_float($variable) && !($variable === null)) {
+            throw new \InvalidArgumentException('provided variable is not a number or null. found: ' . $variable);
         }
     }
 }
diff --git a/test/MathParserTest.php b/test/MathParserTest.php
index a0f5de2..698ac92 100644
--- a/test/MathParserTest.php
+++ b/test/MathParserTest.php
@@ -106,6 +106,7 @@ public static function provideVariableData()
             [['$a1 + $b2 + $0', ['a1' => 0, 'b2' => 41, 0 => 13]], 54],
             [['$0 + $2', [0, 41, 13]], 13],
             [['$0 + $2', [0, 41, 13]], 13],
+            [['$0 + $2', [null, 41, 13]], null],
         ];
         
         return $data;