diff --git a/modules/core/validator.js b/modules/core/validator.js
index d5be80a78..8392ab911 100644
--- a/modules/core/validator.js
+++ b/modules/core/validator.js
@@ -334,7 +334,7 @@ export function validationIssue(attrs) {
     this.type = attrs.type;                // required
     this.severity = attrs.severity;        // required - 'warning' or 'error'
     this.message = attrs.message;          // required - localized string
-    this.reference = attrs.reference;      // required - function(selection) to render reference info
+    this.reference = attrs.reference;      // optional - function(selection) to render reference info
     this.entities = attrs.entities;        // optional - array of entities
     this.loc = attrs.loc;                  // optional - expect a [lon, lat] array
     this.data = attrs.data;                // optional - object containing extra data for the fixes
diff --git a/modules/ui/entity_issues.js b/modules/ui/entity_issues.js
index 352f21da9..4b961c4f0 100644
--- a/modules/ui/entity_issues.js
+++ b/modules/ui/entity_issues.js
@@ -160,8 +160,13 @@ export function uiEntityIssues(context) {
             .style('max-height', '0')
             .style('opacity', '0')
             .each(function(d) {
-                d3_select(this)
-                    .call(d.reference);
+                if (typeof d.reference === 'function') {
+                    d3_select(this)
+                        .call(d.reference);
+                } else {
+                    d3_select(this)
+                        .text(t('inspector.no_documentation_key'));
+                }
             });