diff --git a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/constants.js b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/constants.js
index d02705d01..9e9d43f01 100644
--- a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/constants.js
+++ b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/constants.js
@@ -34,6 +34,8 @@ const mapTypeToDescription = {
 	[PICK_LIST]: 'Generate a sub-list by randomly choosing from a list'
 }
 
+const SERIES_TYPE_OPTIONS = ['', 'arithmetic', 'geometric']
+
 export {
 	STATIC_VALUE,
 	RANDOM_NUMBER,
@@ -42,6 +44,7 @@ export {
 	RANDOM_SEQUENCE,
 	PICK_ONE,
 	PICK_LIST,
+	SERIES_TYPE_OPTIONS,
 	typeList,
 	mapTypeToString,
 	mapTypeToDescription
diff --git a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-list-modal.js b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-list-modal.js
index a4e01f147..855a4cfef 100644
--- a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-list-modal.js
+++ b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-list-modal.js
@@ -6,7 +6,6 @@ import Common from 'obojobo-document-engine/src/scripts/common'
 import VariableProperty from './variable-property/variable-property'
 import NewVariable from './new-variable/new-variable'
 import VariableBlock from './variable-block'
-import { RANDOM_NUMBER, RANDOM_LIST } from './constants'
 import { getParsedRange } from '../../../common/util/range-parsing'
 import { changeVariableToType, validateVariableValue, validateMultipleVariables } from './variable-util'
 
diff --git a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.js b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.js
index 9c4193629..e3922fcbe 100644
--- a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.js
+++ b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.js
@@ -8,7 +8,8 @@ import {
 	RANDOM_LIST,
 	RANDOM_SEQUENCE,
 	PICK_ONE,
-	PICK_LIST
+	PICK_LIST,
+	SERIES_TYPE_OPTIONS
 } from '../constants'
 
 const VariableValues = props => {
@@ -93,6 +94,10 @@ const VariableValues = props => {
 		return className
 	}
 
+	const constrainSeriesTypeOption = value => {
+		return SERIES_TYPE_OPTIONS.includes(value) ? value : ''
+	}
+
 	switch (variable.type) {
 		case STATIC_VALUE:
 			return (
@@ -312,7 +317,7 @@ const VariableValues = props => {
 						<select
 							className={getInputClassForType('seriesType', true)}
 							name="seriesType"
-							value={variable.seriesType || ''}
+							value={constrainSeriesTypeOption(variable.seriesType) || ''}
 							onChange={onChange}
 						>
 							<option disabled value="">
@@ -322,6 +327,13 @@ const VariableValues = props => {
 							<option value="geometric">Geometric (Multiply)</option>
 						</select>
 					</label>
+					{variable.errors
+					&& variable.errors.seriesType
+					&& (! SERIES_TYPE_OPTIONS.includes(variable.seriesType)) ?
+					<span className="invalid-value-warning">
+						Invalid option &quot;{variable.seriesType}&quot;
+					</span>
+					: null}
 
 					<label className="variable-values--group">
 						<label>Step by: </label>
diff --git a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.scss b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.scss
index 32ad207d3..e455c72da 100644
--- a/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.scss
+++ b/packages/app/obojobo-document-engine/src/scripts/oboeditor/components/variables/variable-property/variable-value.scss
@@ -29,4 +29,10 @@
 			margin: 0 0.3em;
 		}
 	}
+	.invalid-value-warning {
+		display: block;
+		margin-left: 11.5em;
+		font-size: 0.75em;
+		color: $color-dangerous;
+	}
 }
diff --git a/packages/app/obojobo-document-engine/src/scripts/viewer/components/nav.js b/packages/app/obojobo-document-engine/src/scripts/viewer/components/nav.js
index 377d7c801..08a615f77 100644
--- a/packages/app/obojobo-document-engine/src/scripts/viewer/components/nav.js
+++ b/packages/app/obojobo-document-engine/src/scripts/viewer/components/nav.js
@@ -108,14 +108,36 @@ export default class Nav extends React.Component {
 		FocusUtil.focusOnNavTarget()
 	}
 
-	renderLabel(label) {
-		return label instanceof StyleableText ? label.value : label
+	renderLabel(label, sourceModel) {
+		return this.substituteLabelVariables((label instanceof StyleableText ? label.value : label), sourceModel)
 	}
 
-	renderLinkButton(label, ariaLabel, isDisabled, refId = null) {
+	substituteLabelVariables(label, sourceModel) {
+		// Quickly exit if the text has no double brackets, since we
+		// know there are no variables to substitue:
+		if (!sourceModel || label.indexOf('{{') === -1) {
+			return label
+		}
+
+		const varRegex = /\{\{(.+?)\}\}/g
+		const match = varRegex.exec(label)
+
+		if (match !== null) {
+			const event = { text: '', isNav: true }
+
+			// Ask the system if anything is aware of `variableText`:
+			Dispatcher.trigger('getTextForVariable', event, match[1], sourceModel)
+			if (event.text !== null) {
+				return event.text
+			}
+		}
+		return label
+	}
+
+	renderLinkButton(label, ariaLabel, isDisabled, sourceModel, refId = null) {
 		return (
 			<button ref={refId} aria-disabled={isDisabled} aria-label={ariaLabel}>
-				{this.renderLabel(label)}
+				{this.renderLabel(label, sourceModel)}
 			</button>
 		)
 	}
@@ -147,7 +169,7 @@ export default class Nav extends React.Component {
 
 		return (
 			<li key={index} onClick={this.onClick.bind(this, item)} className={className}>
-				{this.renderLinkButton(item.label, ariaLabel, isItemDisabled, item.id)}
+				{this.renderLinkButton(item.label, ariaLabel, isItemDisabled, item.sourceModel, item.id)}
 				{item.flags.assessment ? (
 					<span className="assessment-info">{this.getAssessmentInfo(item)}</span>
 				) : null}
@@ -175,7 +197,7 @@ export default class Nav extends React.Component {
 
 		return (
 			<li key={index} onClick={this.onClick.bind(this, item)} className={className}>
-				{this.renderLinkButton(item.label, ariaLabel, isItemDisabled)}
+				{this.renderLinkButton(item.label, ariaLabel, isItemDisabled, item.sourceModel)}
 				{lockEl}
 			</li>
 		)
diff --git a/packages/app/obojobo-document-engine/src/scripts/viewer/stores/nav-store.js b/packages/app/obojobo-document-engine/src/scripts/viewer/stores/nav-store.js
index 341035ccb..74794132c 100644
--- a/packages/app/obojobo-document-engine/src/scripts/viewer/stores/nav-store.js
+++ b/packages/app/obojobo-document-engine/src/scripts/viewer/stores/nav-store.js
@@ -351,7 +351,8 @@ class NavStore extends Store {
 				path: '',
 				showChildren: true,
 				showChildrenOnNavigation: true,
-				parent: null
+				parent: null,
+				sourceModel: model
 			},
 			navItem
 		)