diff --git a/demo-app/src/stories/TimelineScenarios.ts b/demo-app/src/stories/TimelineScenarios.ts
index 5f5814e3..4bc56136 100644
--- a/demo-app/src/stories/TimelineScenarios.ts
+++ b/demo-app/src/stories/TimelineScenarios.ts
@@ -12,4 +12,5 @@ export const timelineScenarios = {
   propertyMinDate: 'PROPERTY minDate',
   propertyMaxDate: 'PROPERTY maxDate',
   propertyShowZoomShortcuts: 'PROPERTY showZoomShortcuts',
+  givenIlegalDataThenShowWarning: "GIVEN illegal data, THEN show warning icon",
 };
diff --git a/demo-app/src/stories/basic/Basic.stories.tsx b/demo-app/src/stories/basic/Basic.stories.tsx
index 38b1c354..6206cbc0 100644
--- a/demo-app/src/stories/basic/Basic.stories.tsx
+++ b/demo-app/src/stories/basic/Basic.stories.tsx
@@ -136,4 +136,15 @@ HorizontalScroll.parameters = {
     timelineScenarios.propertyMinDate,
     timelineScenarios.propertyMaxDate
   ]
-};
\ No newline at end of file
+};
+
+export const IllegalData = () => {
+
+  return (<Timeline startDate={d('2018-09-20')} endDate={d('2018-09-21')} groups={[someHumanResources[0]]} items={someTasks} />);
+};
+
+IllegalData.parameters = {
+  scenarios: [
+    timelineScenarios.givenIlegalDataThenShowWarning
+  ]
+}
diff --git a/src/style.css b/src/style.css
index 30c3cdc7..72f9b1b1 100644
--- a/src/style.css
+++ b/src/style.css
@@ -414,6 +414,12 @@
   top: 1px;
 }
 
+.rct9k-warning-div {
+  position: absolute;
+  right: 30px;
+  top: 3px;
+}
+
 .rct9k-menu.ui.popup {
   padding: 0 !important;
   margin: 0 !important;
diff --git a/src/timeline.js b/src/timeline.js
index 9488337c..bf643367 100644
--- a/src/timeline.js
+++ b/src/timeline.js
@@ -7,7 +7,7 @@ import Measure from 'react-measure';
 
 import interact from 'interactjs';
 import _ from 'lodash';
-import {Button, Popup} from 'semantic-ui-react';
+import {Button, Popup, Icon} from 'semantic-ui-react';
 import {Group, InteractOption, Item, RowLayer} from './index';
 
 import {TestsAreDemoCheat, createTestids} from '@famiprog-foundation/tests-are-demo';
@@ -613,7 +613,8 @@ export default class Timeline extends React.Component {
       touchPositionX: undefined,
       fadeEffectOpen: false,
       fadeEffectContent: undefined,
-      fadeEffectOpacity: 0
+      fadeEffectOpacity: 0,
+      illegalInputData: false
     };
 
     // These functions need to be bound because they are passed as parameters.
@@ -1036,6 +1037,7 @@ export default class Timeline extends React.Component {
 
     let fillInGroups = [];
 
+    let illegalInputData = false;
     let groupId = groups.length;
     while (heightToFillIn > 0) {
       // create new empty group;
@@ -1047,10 +1049,17 @@ export default class Timeline extends React.Component {
       }
 
       fillInGroups.push(emptyGroup);
+      if (this.rowItemMap[groupId] && this.rowItemMap[groupId].length) {
+        const itemKeys = this.rowItemMap[groupId].map(item => item.key).join(', ');
+        console.error(
+          `The items with keys: [${itemKeys}] has row = ${groupId} but there's no group with index ${groupId}. Please check if the groups property or the items with keys [${itemKeys}] are well defined.`
+        );
+        illegalInputData = true;
+      }
       heightToFillIn -= this.props.itemHeight;
       groupId++;
     }
-    this.setState({groups: [...groups, ...fillInGroups]});
+    this.setState({groups: [...groups, ...fillInGroups], illegalInputData});
   }
 
   /**
@@ -2025,6 +2034,17 @@ export default class Timeline extends React.Component {
     }
   }
 
+  /**
+   * @returns { JSX.Element }
+   */
+  renderWarningIcon() {
+    return (
+      <Popup position="top right" trigger={<Icon color="red" size="big" name="exclamation" />}>
+        <div style={{color: 'red'}}> Some of the input data are illegal. More info in the console.</div>
+      </Popup>
+    );
+  }
+
   /**
    * @returns { JSX.Element }
    */
@@ -2336,6 +2356,7 @@ export default class Timeline extends React.Component {
                       topOffset: timebarHeight,
                       verticalGridLines: this.state.verticalGridLines
                     })}
+                  {this.state.illegalInputData && <div className="rct9k-warning-div">{this.renderWarningIcon()}</div>}
                   <div className="rct9k-menu-div">{this.renderMenuButton()}</div>
                 </div>
               </div>