"Invalid type: Is" when using "is defined"

[GreenGremlin]

When using "is defined", I'm getting "Template render error: assertType: invalid type: Is".

{% if content is defined %}
    {{ content }}
{% endif %}```

Full error output

Template render error: Template render error: assertType: invalid type: Is
at Object._prettifyError (/Users/jonathan/testApp/node_modules/nunjucks/src/lib.js:35:11)
at /Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:547:19
at Template.root [as rootRenderFunc] (eval at _compile (/Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:617:18), :22:3)
at Template.render (/Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:540:10)
at /Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:350:27
at createTemplate (/Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:299:9)
at handle (/Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:311:11)
at /Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:323:9
at next (/Users/jonathan/testApp/node_modules/nunjucks/src/lib.js:281:7)
at Object.asyncIter (/Users/jonathan/testApp/node_modules/nunjucks/src/lib.js:287:3)
at Environment.getTemplate (/Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:305:9)
at Environment.render (/Users/jonathan/testApp/node_modules/nunjucks/src/environment.js:344:10)
at Object.render (/Users/jonathan/testApp/node_modules/nunjucks/index.js:91:14)
at Object.processStyles (/Users/jonathan/testApp/utils/css.js:220:27)
at main (/Users/jonathan/testApp/main.js:16:33)
at

I'm using Nunjucks 3.1.2