diff --git a/core/modules/filters.js b/core/modules/filters.js
index 5f4eb6492..808628acb 100644
--- a/core/modules/filters.js
+++ b/core/modules/filters.js
@@ -247,8 +247,8 @@ exports.compileFilter = function(filterString) {
 					// Use the "title" operator if no operator is specified
 					operatorFunction = filterOperators.title;
 				} else if(!filterOperators[operator.operator]) {
-					// Unknown operators treated as "unknown" - at run time we can distinguish between a custom operator and falling back to the default "field" operator
-					operatorFunction = filterOperators.unknown;
+					// Unknown operators treated as "[unknown]" - at run time we can distinguish between a custom operator and falling back to the default "field" operator
+					operatorFunction = filterOperators["[unknown]"];
 				} else {
 					// Use the operator function
 					operatorFunction = filterOperators[operator.operator];
diff --git a/core/modules/filters/unknown.js b/core/modules/filters/unknown.js
index c2e2912a4..2eca09a7c 100644
--- a/core/modules/filters/unknown.js
+++ b/core/modules/filters/unknown.js
@@ -3,7 +3,9 @@ title: $:/core/modules/filters/unknown.js
 type: application/javascript
 module-type: filteroperator
 
-Filter operator for handling unknown filter operators
+Filter operator for handling unknown filter operators.
+
+Not intended to be used directly by end users, hence the square brackets around the name.
 
 \*/
 (function(){
@@ -17,7 +19,7 @@ var fieldFilterOperatorFn = require("$:/core/modules/filters/field.js").field;
 /*
 Export our filter function
 */
-exports.unknown = function(source,operator,options) {
+exports["[unknown]"] = function(source,operator,options) {
 	var customDefinitionTitle = "[" + operator.operator + "[]]",
 		customDefinition = options.widget && options.widget.getVariableInfo && options.widget.getVariableInfo(customDefinitionTitle);
 	if(customDefinition && customDefinition.srcVariable && customDefinition.srcVariable.isFunctionDefinition) {