Panic when using Inline function

_time=2019-07-10T15:40:17.022-07:00 _level=SEVERE _msg=panic: interface conversion: interface is nil, not plan.Operator

_time=2019-07-10T15:40:17.022-07:00 _level=SEVERE _msg=request text: <ud>execute function avg_test(10)</ud>

_time=2019-07-10T15:40:17.022-07:00 _level=SEVERE _msg=stack: goroutine 1868 [running]:

github.com/couchbase/query/execution.(*Context).Recover(0xc000404f00, 0xc001062a00)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/context.go:629 +0xbc

panic(0x1c94c20, 0xc0015d74d0)

	/home/couchbase/.cbdepscache/exploded/x86_64/go-1.11.4/go/src/runtime/panic.go:513 +0x1b9

github.com/couchbase/query/execution.(*Context).EvaluateSubquery(0xc000404f00, 0xc0015d4840, 0x20c6f40, 0xc0015d7320, 0xc000404f00, 0x1e9d900, 0x0, 0x20af640)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/context.go:509 +0x149

github.com/couchbase/query/algebra.(*Subquery).Evaluate(0xc0015d6c30, 0x20c6f40, 0xc0015d7320, 0x20af640, 0xc000404f00, 0x10, 0xc001139d20, 0xc517a1, 0x1f4cbc8)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/algebra/subquery.go:68 +0x7b

github.com/couchbase/query/functions/inline.(*inline).Execute(0x335e9f8, 0x20b62a0, 0xc00177cc00, 0x20b0040, 0xc0015d6c60, 0x1, 0xc0018f34c0, 0x1, 0x1, 0x20af680, ...)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/functions/inline/inline.go:59 +0x247

github.com/couchbase/query/functions.ExecuteFunction(0x20b62a0, 0xc00177cc00, 0x1, 0xc0018f34c0, 0x1, 0x1, 0x20af680, 0xc000404f00, 0x0, 0xc000a5f7a8, ...)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/functions/functions.go:394 +0x247

github.com/couchbase/query/execution.(*ExecuteFunction).RunOnce.func1()

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/function_execute.go:78 +0x1e0

github.com/couchbase/query/util.(*Once).Do(0xc001062b08, 0xc000a5f788)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/util/sync.go:54 +0x4a

github.com/couchbase/query/execution.(*ExecuteFunction).RunOnce(0xc001062a00, 0xc000404f00, 0x0, 0x0)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/function_execute.go:47 +0x82

created by github.com/couchbase/query/execution.(*Authorize).RunOnce.func1

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/authorize.go:90 +0x2b2

goroutine 1868 [running]:

github.com/couchbase/query/execution.(*Context).Recover(0xc000404f00, 0xc001062a00)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/context.go:629 +0xbc

panic(0x1c94c20, 0xc0015d74d0)

	/home/couchbase/.cbdepscache/exploded/x86_64/go-1.11.4/go/src/runtime/panic.go:513 +0x1b9

github.com/couchbase/query/execution.(*Context).EvaluateSubquery(0xc000404f00, 0xc0015d4840, 0x20c6f40, 0xc0015d7320, 0xc000404f00, 0x1e9d900, 0x0, 0x20af640)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/context.go:509 +0x149

github.com/couchbase/query/algebra.(*Subquery).Evaluate(0xc0015d6c30, 0x20c6f40, 0xc0015d7320, 0x20af640, 0xc000404f00, 0x10, 0xc001139d20, 0xc517a1, 0x1f4cbc8)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/algebra/subquery.go:68 +0x7b

github.com/couchbase/query/functions/inline.(*inline).Execute(0x335e9f8, 0x20b62a0, 0xc00177cc00, 0x20b0040, 0xc0015d6c60, 0x1, 0xc0018f34c0, 0x1, 0x1, 0x20af680, ...)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/functions/inline/inline.go:59 +0x247

github.com/couchbase/query/functions.ExecuteFunction(0x20b62a0, 0xc00177cc00, 0x1, 0xc0018f34c0, 0x1, 0x1, 0x20af680, 0xc000404f00, 0x0, 0xc000a5f7a8, ...)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/functions/functions.go:394 +0x247

github.com/couchbase/query/execution.(*ExecuteFunction).RunOnce.func1()

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/function_execute.go:78 +0x1e0

github.com/couchbase/query/util.(*Once).Do(0xc001062b08, 0xc000a5f788)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/util/sync.go:54 +0x4a

github.com/couchbase/query/execution.(*ExecuteFunction).RunOnce(0xc001062a00, 0xc000404f00, 0x0, 0x0)

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/function_execute.go:47 +0x82

created by github.com/couchbase/query/execution.(*Authorize).RunOnce.func1

	/home/couchbase/jenkins/workspace/couchbase-server-unix/goproj/src/github.com/couchbase/query/execution/authorize.go:90 +0x2b2