Undefined method `http' when running Inspec via Jenkins pipeline

I am quite new to Inspec & Ruby. I've hit my first blocker that I'm struggling to troubleshoot solo.

I have created a number of Inspec tests using both the host method and the http method. host works fine both in my local environment and via a devbox going through my Jenkins pipeline. However, when I try to run a test using the http method, it doesn't recognise it and Jenkins throws me back a failed exit code 100.

Example of one test which fails:

control "cis-f5-benchmark-1.1.3" do                                                                     
    impact 1.0                                                                                        
    title "Configure Secure Password Policy (Manual)"                                                   
    describe json(content: http("https://#{BIGIP_HOST}:#{BIGIP_PORT}/mgmt/tm/auth/password-policy",
              auth: {user: BIGIP_USER, pass: BIGIP_PASSWORD},
              method: 'GET',
              ssl_verify: false).body) do
          its('policyEnforcement') { should eq 'enabled' }
          its('minimumLength') { should eq 14 }
          its('requiredSpecial') { should eq 3 }
          its('requiredUppercase') { should eq 3 }
          its('requiredLowercase') { should eq 3 }
          its('requiredNumeric') { should eq 3 }
          its('minDuration') { should eq 0 }
          its('maxDuration') { should eq 99999 }
          its('passwordMemory') { should eq 5 }
          its('maxLoginFailures') { should eq 3 }