diff --git a/inc/caldav-REPORT-calquery.php b/inc/caldav-REPORT-calquery.php
index 4bdf7f9a..f41c3979 100644
--- a/inc/caldav-REPORT-calquery.php
+++ b/inc/caldav-REPORT-calquery.php
@@ -86,13 +86,16 @@ function SqlFilterFragment( $filter, $components, $property = null, $parameter =
     dbg_error_log("calquery", "Processing $tag into SQL - %d, '%s', %d\n", count($components), $property, isset($parameter) );
 
     $not_defined = "";
+    $not_defined_log = "";
     switch( $tag ) {
       case 'urn:ietf:params:xml:ns:caldav:is-not-defined':
-        $not_defined = "not"; // then fall through to IS-DEFINED case
+        $not_defined = "not ";     // for SQL
+        $not_defined_log = "not-"; // for logging
+        // then fall through to IS-DEFINED case
       case 'urn:ietf:params:xml:ns:caldav:is-defined':
         if ( isset( $parameter ) ) {
           $need_post_filter = true;
-          dbg_error_log("calquery", "Could not handle 'is-%sdefined' on property %s, parameter %s in SQL", $not_defined, $property, $parameter );
+          dbg_error_log("calquery", "Could not handle 'is-%sdefined' on property %s, parameter %s in SQL", $not_defined_log, $property, $parameter );
           return false;  // Not handled in SQL
         }
         if ( isset( $property ) ) {
@@ -114,7 +117,7 @@ function SqlFilterFragment( $filter, $components, $property = null, $parameter =
             default:
               $property_defined_match = "LIKE '_%'";  // i.e. contains a single character or more
           }
-          $sql .= sprintf( "AND %s %s %s ", $property, $not_defined, $property_defined_match );
+          $sql .= sprintf( "AND %s %s%s ", $property, $not_defined, $property_defined_match );
         }
         break;
 
diff --git a/testing/tests/regression-suite/0966-REPORT-not-defined.result b/testing/tests/regression-suite/0966-REPORT-not-defined.result
new file mode 100644
index 00000000..a9bee7e1
--- /dev/null
+++ b/testing/tests/regression-suite/0966-REPORT-not-defined.result
@@ -0,0 +1,10 @@
+HTTP/1.1 207 Multi-Status
+Date: Dow, 01 Jan 2000 00:00:00 GMT
+DAV: 1, 2, 3, access-control, calendar-access, calendar-schedule
+DAV: extended-mkcol, bind, addressbook, calendar-auto-schedule, calendar-proxy
+ETag: "07474790757c5e1b526ce4901889d6d3"
+Content-Length: 68
+Content-Type: text/xml; charset="utf-8"
+
+<?xml version="1.0" encoding="utf-8" ?>
+<multistatus xmlns="DAV:"/>
diff --git a/testing/tests/regression-suite/0966-REPORT-not-defined.test b/testing/tests/regression-suite/0966-REPORT-not-defined.test
new file mode 100644
index 00000000..679f9202
--- /dev/null
+++ b/testing/tests/regression-suite/0966-REPORT-not-defined.test
@@ -0,0 +1,37 @@
+#
+# not-defined REPORT 
+#
+# Regression test for https://gitlab.com/davical-project/davical/-/issues/279
+#
+# Returns nothing, but at least we don't get a fatal error now.
+# XXX: Changed to be a test that returns records
+#
+TYPE=REPORT
+URL=http://regression.host/caldav.php/user1/home/
+HEADER=Content-Type: text/xml; charset="UTF-8"
+HEADER=Depth: 0
+HEAD
+
+
+BEGINDATA
+<?xml version='1.0' encoding='utf-8'?>
+<C:calendar-query xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
+  <D:prop>
+    <C:calendar-data/>
+  </D:prop>
+  <C:filter>
+    <C:comp-filter name="VCALENDAR">
+      <C:comp-filter name="VTODO">
+        <C:prop-filter name="COMPLETED">
+          <C:is-not-defined/>
+        </C:prop-filter>
+        <C:prop-filter name="STATUS">
+          <C:is-not-defined/>
+        </C:prop-filter>
+      </C:comp-filter>
+    </C:comp-filter>
+  </C:filter>
+</C:calendar-query>
+ENDDATA
+
+