diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index 91271e1..7c4d94c 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -799,7 +799,14 @@ class Executor
     private static function completeLeafValue(Type $returnType, &$result)
     {
         Utils::invariant(method_exists($returnType, 'serialize'), 'Missing serialize method on type');
-        return $returnType->serialize($result);
+        $serializedResult = $returnType->serialize($result);
+
+        if ($serializedResult === null) {
+            throw new \UnexpectedValueException(
+                'Expected a value of type "'. $returnType . '" but received: ' . Utils::printSafe($result)
+            );
+        }
+        return $serializedResult;
     }
 
     /**
diff --git a/src/Type/Definition/FloatType.php b/src/Type/Definition/FloatType.php
index 7beb1df..522d507 100644
--- a/src/Type/Definition/FloatType.php
+++ b/src/Type/Definition/FloatType.php
@@ -3,6 +3,7 @@ namespace GraphQL\Type\Definition;
 
 use GraphQL\Language\AST\FloatValue;
 use GraphQL\Language\AST\IntValue;
+use GraphQL\Utils;
 
 /**
  * Class FloatType
@@ -47,7 +48,15 @@ values as specified by
      */
     private function coerceFloat($value)
     {
-        return is_numeric($value) || $value === true || $value === false ? (float) $value : null;
+        if ($value === '') {
+            throw new \UnexpectedValueException(
+                'Float cannot represent non numeric value: (empty string)'
+            );
+        }
+        if (is_numeric($value) || $value === true || $value === false) {
+            return (float)$value;
+        }
+        throw new \UnexpectedValueException('Float cannot represent non numeric value: ' . Utils::printSafe($value));
     }
 
     /**
diff --git a/src/Type/Definition/IntType.php b/src/Type/Definition/IntType.php
index 77a04ed..b36cca8 100644
--- a/src/Type/Definition/IntType.php
+++ b/src/Type/Definition/IntType.php
@@ -3,6 +3,7 @@ namespace GraphQL\Type\Definition;
 
 use GraphQL\Language\AST\IntValue;
 use GraphQL\Language\AST\Value;
+use GraphQL\Utils;
 
 /**
  * Class IntType
@@ -54,13 +55,20 @@ values. Int can represent values between -(2^31) and 2^31 - 1. ';
      */
     private function coerceInt($value)
     {
+        if ($value === '') {
+            throw new \UnexpectedValueException(
+                'Int cannot represent non 32-bit signed integer value: (empty string)'
+            );
+        }
         if (false === $value || true === $value) {
             return (int) $value;
         }
         if (is_numeric($value) && $value <= self::MAX_INT && $value >= self::MIN_INT) {
             return (int) $value;
         }
-        return null;
+        throw new \UnexpectedValueException(
+            'Int cannot represent non 32-bit signed integer value: ' . Utils::printSafe($value)
+        );
     }
 
     /**
diff --git a/tests/Type/ScalarSerializationTest.php b/tests/Type/ScalarSerializationTest.php
index 21f0864..cd95daf 100644
--- a/tests/Type/ScalarSerializationTest.php
+++ b/tests/Type/ScalarSerializationTest.php
@@ -15,6 +15,7 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
         $intType = Type::int();
 
         $this->assertSame(1, $intType->serialize(1));
+        $this->assertSame(123, $intType->serialize('123'));
         $this->assertSame(0, $intType->serialize(0));
         $this->assertSame(-1, $intType->serialize(-1));
         $this->assertSame(0, $intType->serialize(0.1));
@@ -23,12 +24,50 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
         $this->assertSame(100000, $intType->serialize(1e5));
         // Maybe a safe PHP int, but bigger than 2^32, so not
         // representable as a GraphQL Int
-        $this->assertSame(null, $intType->serialize(9876504321));
-        $this->assertSame(null, $intType->serialize(-9876504321));
-        $this->assertSame(null, $intType->serialize(1e100));
-        $this->assertSame(null, $intType->serialize(-1e100));
+        try {
+            $intType->serialize(9876504321);
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Int cannot represent non 32-bit signed integer value: 9876504321', $e->getMessage());
+        }
+
+        try {
+            $intType->serialize(-9876504321);
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Int cannot represent non 32-bit signed integer value: -9876504321', $e->getMessage());
+        }
+
+        try {
+            $intType->serialize(1e100);
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Int cannot represent non 32-bit signed integer value: 1.0E+100', $e->getMessage());
+        }
+
+        try {
+            $intType->serialize(-1e100);
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Int cannot represent non 32-bit signed integer value: -1.0E+100', $e->getMessage());
+        }
+
         $this->assertSame(-1, $intType->serialize('-1.1'));
-        $this->assertSame(null, $intType->serialize('one'));
+
+        try {
+            $intType->serialize('one');
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Int cannot represent non 32-bit signed integer value: one', $e->getMessage());
+        }
+
+        try {
+            $intType->serialize('');
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Int cannot represent non 32-bit signed integer value: (empty string)', $e->getMessage());
+        }
+
         $this->assertSame(0, $intType->serialize(false));
         $this->assertSame(1, $intType->serialize(true));
     }
@@ -42,12 +81,27 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
 
         $this->assertSame(1.0, $floatType->serialize(1));
         $this->assertSame(0.0, $floatType->serialize(0));
+        $this->assertSame(123.5, $floatType->serialize('123.5'));
         $this->assertSame(-1.0, $floatType->serialize(-1));
         $this->assertSame(0.1, $floatType->serialize(0.1));
         $this->assertSame(1.1, $floatType->serialize(1.1));
         $this->assertSame(-1.1, $floatType->serialize(-1.1));
         $this->assertSame(-1.1, $floatType->serialize('-1.1'));
-        $this->assertSame(null, $floatType->serialize('one'));
+
+        try {
+            $floatType->serialize('one');
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Float cannot represent non numeric value: one', $e->getMessage());
+        }
+
+        try {
+            $floatType->serialize('');
+            $this->fail('Expected exception was not thrown');
+        } catch (\UnexpectedValueException $e) {
+            $this->assertEquals('Float cannot represent non numeric value: (empty string)', $e->getMessage());
+        }
+
         $this->assertSame(0.0, $floatType->serialize(false));
         $this->assertSame(1.0, $floatType->serialize(true));
     }
@@ -81,6 +135,6 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
         $this->assertSame(true, $boolType->serialize(true));
         $this->assertSame(false, $boolType->serialize(false));
 
-        // TODO: how should it behaive on '0'?
+        // TODO: how should it behave on '0'?
     }
 }