diff --git a/AbuseFilter.class.php b/AbuseFilter.class.php
index df207d8ad..6c9958476 100644
--- a/AbuseFilter.class.php
+++ b/AbuseFilter.class.php
@@ -1187,7 +1187,13 @@ class AbuseFilter {
 				$block->prevents( 'createaccount', true );
 				$block->prevents( 'editownusertalk', false );
 
-				$expiry = $wgUser->isAnon() ? $wgAbuseFilterAnonBlockDuration : $wgAbuseFilterBlockDuration;
+				if ( $wgUser->isAnon() && $wgAbuseFilterAnonBlockDuration !== null ) {
+					// The user isn't logged in and the anon block duration doesn't default to $wgAbuseFilterBlockDuration
+					$expiry = $wgAbuseFilterAnonBlockDuration;
+				} else {
+					$expiry = $wgAbuseFilterBlockDuration;
+				}
+
 				$block->mExpiry = SpecialBlock::parseExpiryInput( $expiry );
 				$block->insert();
 
diff --git a/AbuseFilter.php b/AbuseFilter.php
index 81391db82..556c44d97 100644
--- a/AbuseFilter.php
+++ b/AbuseFilter.php
@@ -187,8 +187,8 @@ $wgAbuseFilterDisallowGlobalLocalBlocks = false;
 
 // Block duration for logged in users
 $wgAbuseFilterBlockDuration = 'indefinite';
-// Block duration for anonymous users
-$wgAbuseFilterAnonBlockDuration = $wgAbuseFilterBlockDuration;
+// Block duration for anonymous users ($wgAbuseFilterBlockDuration will be used if null)
+$wgAbuseFilterAnonBlockDuration = null;
 
 // Callback functions for custom actions
 $wgAbuseFilterCustomActionsHandlers = array();