diff --git a/includes/parser/AFPData.php b/includes/parser/AFPData.php
index 79288a3b1..8f46f4465 100644
--- a/includes/parser/AFPData.php
+++ b/includes/parser/AFPData.php
@@ -58,7 +58,7 @@ class AFPData {
 	 */
 	public function __construct( $type, $val = null ) {
 		if ( ( $type === self::DUNDEFINED || $type === self::DEMPTY ) && $val !== null ) {
-			// Sanity, and we rely on this for instance when casting a DUNDEFINED to something else
+			// Sanity
 			throw new InvalidArgumentException( 'DUNDEFINED cannot have a non-null value' );
 		}
 		$this->type = $type;
@@ -111,6 +111,11 @@ class AFPData {
 		if ( $orig->type === $target ) {
 			return $orig->dup();
 		}
+		if ( $orig->type === self::DUNDEFINED ) {
+			// This case should be handled at a higher level, to avoid implicitly relying on what
+			// this method will do for the specific case.
+			throw new AFPException( 'Refusing to cast DUNDEFINED to something else' );
+		}
 		if ( $target === self::DNULL ) {
 			// We don't expose any method to cast to null. And, actually, should we?
 			return new AFPData( self::DNULL );
@@ -340,8 +345,9 @@ class AFPData {
 	 * @throws AFPException
 	 */
 	public static function boolOp( AFPData $a, AFPData $b, $op ) {
-		$a = $a->toBool();
-		$b = $b->toBool();
+		$a = $a->type === self::DUNDEFINED ? false : $a->toBool();
+		$b = $b->type === self::DUNDEFINED ? false : $b->toBool();
+
 		if ( $op === '|' ) {
 			return new AFPData( self::DBOOL, $a || $b );
 		} elseif ( $op === '&' ) {
diff --git a/includes/parser/AbuseFilterCachingParser.php b/includes/parser/AbuseFilterCachingParser.php
index f3be7a913..159248156 100644
--- a/includes/parser/AbuseFilterCachingParser.php
+++ b/includes/parser/AbuseFilterCachingParser.php
@@ -63,9 +63,14 @@ class AbuseFilterCachingParser extends AbuseFilterParser {
 			}
 		);
 
-		return $tree
+		$res = $tree
 			? $this->evalNode( $tree )
 			: new AFPData( AFPData::DNULL, null );
+
+		if ( $res->getType() === AFPData::DUNDEFINED ) {
+			$res = new AFPData( AFPData::DBOOL, false );
+		}
+		return $res;
 	}
 
 	/**
@@ -245,7 +250,7 @@ class AbuseFilterCachingParser extends AbuseFilterParser {
 			case AFPTreeNode::LOGIC:
 				list( $op, $leftOperand, $rightOperand ) = $node->children;
 				$leftOperand = $this->evalNode( $leftOperand );
-				$value = $leftOperand->toBool();
+				$value = $leftOperand->getType() === AFPData::DUNDEFINED ? false : $leftOperand->toBool();
 				// Short-circuit.
 				if ( ( !$value && $op === '&' ) || ( $value && $op === '|' ) ) {
 					if ( $rightOperand instanceof AFPTreeNode ) {
diff --git a/includes/parser/AbuseFilterParser.php b/includes/parser/AbuseFilterParser.php
index 31469ecbd..b2c2facf2 100644
--- a/includes/parser/AbuseFilterParser.php
+++ b/includes/parser/AbuseFilterParser.php
@@ -307,6 +307,10 @@ class AbuseFilterParser {
 		$result = new AFPData( AFPData::DEMPTY );
 		$this->doLevelEntry( $result );
 
+		if ( $result->getType() === AFPData::DUNDEFINED ) {
+			$result = new AFPData( AFPData::DBOOL, false );
+		}
+
 		return $result;
 	}
 
@@ -560,9 +564,10 @@ class AbuseFilterParser {
 			$op = $this->mCur->value;
 			$this->move();
 			$r2 = new AFPData( AFPData::DEMPTY );
+			$curVal = $result->getType() === AFPData::DUNDEFINED ? false : $result->toBool();
 
 			// We can go on quickly as either one statement with | is true or one with & is false
-			if ( ( $op === '&' && !$result->toBool() ) || ( $op === '|' && $result->toBool() ) ) {
+			if ( ( $op === '&' && !$curVal ) || ( $op === '|' && $curVal ) ) {
 				$orig = $this->mShortCircuit;
 				$this->mShortCircuit = $this->mAllowShort;
 				$this->doLevelCompares( $r2 );
@@ -570,7 +575,7 @@ class AbuseFilterParser {
 					$this->logEmptyOperand( 'bool operand', __METHOD__ );
 				}
 				$this->mShortCircuit = $orig;
-				$result = new AFPData( AFPData::DBOOL, $result->toBool() );
+				$result = new AFPData( AFPData::DBOOL, $curVal );
 				continue;
 			}