diff --git a/index/scorch/segment/zap/new.go b/index/scorch/segment/zap/new.go
index dd2740fb..68827f0b 100644
--- a/index/scorch/segment/zap/new.go
+++ b/index/scorch/segment/zap/new.go
@@ -17,6 +17,7 @@ package zap
 import (
 	"bytes"
 	"encoding/binary"
+	"fmt"
 	"math"
 	"sort"
 	"sync"
@@ -288,6 +289,16 @@ func (s *interim) getOrDefineField(fieldName string) int {
 	return int(fieldIDPlus1 - 1)
 }
 
+// the fieldName must be for a known field
+func (s *interim) getField(fieldName string) int {
+	fieldIDPlus1, exists := s.FieldsMap[fieldName]
+	if !exists || fieldIDPlus1 <= 0 {
+		panic(fmt.Sprintf("getField saw unknown fieldName: %s, fieldsMap: %#v",
+			fieldName, s.FieldsMap))
+	}
+	return int(fieldIDPlus1 - 1)
+}
+
 // fill Dicts and DictKeys from analysis results
 func (s *interim) prepareDicts() {
 	var pidNext int
@@ -328,14 +339,14 @@ func (s *interim) prepareDicts() {
 	for _, result := range s.results {
 		// walk each composite field
 		for _, field := range result.Document.CompositeFields {
-			fieldID := uint16(s.getOrDefineField(field.Name()))
+			fieldID := uint16(s.getField(field.Name()))
 			_, tf := field.Analyze()
 			visitField(fieldID, tf)
 		}
 
 		// walk each field
 		for i, field := range result.Document.Fields {
-			fieldID := uint16(s.getOrDefineField(field.Name()))
+			fieldID := uint16(s.getField(field.Name()))
 			tf := result.Analyzed[i]
 			visitField(fieldID, tf)
 		}
@@ -439,14 +450,14 @@ func (s *interim) processDocument(docNum uint64,
 
 	// walk each composite field
 	for _, field := range result.Document.CompositeFields {
-		fieldID := uint16(s.getOrDefineField(field.Name()))
+		fieldID := uint16(s.getField(field.Name()))
 		ln, tf := field.Analyze()
 		visitField(fieldID, field.Name(), ln, tf)
 	}
 
 	// walk each field
 	for i, field := range result.Document.Fields {
-		fieldID := uint16(s.getOrDefineField(field.Name()))
+		fieldID := uint16(s.getField(field.Name()))
 		ln := result.Length[i]
 		tf := result.Analyzed[i]
 		visitField(fieldID, field.Name(), ln, tf)
@@ -477,7 +488,7 @@ func (s *interim) processDocument(docNum uint64,
 				for _, loc := range tf.Locations {
 					var locf = uint16(fieldID)
 					if loc.Field != "" {
-						locf = uint16(s.getOrDefineField(loc.Field))
+						locf = uint16(s.getField(loc.Field))
 					}
 					var arrayposs []uint64
 					if len(loc.ArrayPositions) > 0 {
@@ -517,7 +528,7 @@ func (s *interim) writeStoredFields() (
 		}
 
 		for _, field := range result.Document.Fields {
-			fieldID := uint16(s.getOrDefineField(field.Name()))
+			fieldID := uint16(s.getField(field.Name()))
 
 			opts := field.Options()