diff --git a/src/BooleanParser.php b/src/BooleanParser.php
index e333a07..16f4b4e 100644
--- a/src/BooleanParser.php
+++ b/src/BooleanParser.php
@@ -6,7 +6,13 @@ class BooleanParser extends AbstractValueParser
 {
     protected function parse($value)
     {
-        return strtoupper($value) === 'TRUE' || strtoupper($value) === '1';
+        if (strtoupper($value) === 'TRUE' || $value === '1') {
+            return true;
+        }
+        if (strtoupper($value) === 'FALSE' || $value === '0') {
+            return false;
+        }
+        return null;
     }
 
     /**
diff --git a/src/YesNoBooleanParser.php b/src/YesNoBooleanParser.php
index f19993b..486607c 100644
--- a/src/YesNoBooleanParser.php
+++ b/src/YesNoBooleanParser.php
@@ -6,7 +6,13 @@ class YesNoBooleanParser extends AbstractValueParser
 {
     protected function parse($value)
     {
-        return strtoupper($value) === 'YES' || strtoupper($value) === 'Y';
+        if (strtoupper($value) === 'YES' || strtoupper($value) === 'Y') {
+            return true;
+        }
+        if (strtoupper($value) === 'NO' || strtoupper($value) === 'N') {
+            return false;
+        }
+        return null;
     }
 
     /**