diff --git a/.circleci/config.yml b/.circleci/config.yml
index 7a01b6f..4cf1ee7 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -30,3 +30,4 @@ jobs:
       - run: curl https://raw.githubusercontent.com/satori/go.uuid/b061729afc07e77a8aa4fad0a2fd840958f1942a/uuid.go > /go/src/github.com/satori/go.uuid/uuid.go
       - run: go test -v ./...
       - run: go run tools/wstest.go `find features -name '*.feature'`
+      - run: go build -tags=gofuzz ./...
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..bfa02df
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,11 @@
+.DS_Store
+
+# ignore generated corpus
+fuzz/corpus/*
+!fuzz/corpus/corpus_*
+fuzz/crashers/*
+fuzz/suppressions/*
+
+# ignore generated fuzz bin
+worksheets-fuzz.zip
+
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..c86e0c1
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,9 @@
+fuzz: clean
+	go-fuzz-build github.com/helloeave/worksheets/fuzz
+	go-fuzz -bin=./worksheets-fuzz.zip -workdir=fuzz/
+
+clean:
+	rm -f worksheets-fuzz.zip
+	find fuzz/corpus/ -type f -not -name 'corpus_[0-9]*' -exec rm {} \;
+	rm -Rf fuzz/crashers/
+	rm -Rf fuzz/suppressions/
diff --git a/fuzz/corpus/corpus_000 b/fuzz/corpus/corpus_000
new file mode 100644
index 0000000..8786692
--- /dev/null
+++ b/fuzz/corpus/corpus_000
@@ -0,0 +1 @@
+worksheet simplest {}
\ No newline at end of file
diff --git a/fuzz/corpus/corpus_001 b/fuzz/corpus/corpus_001
new file mode 100644
index 0000000..5ebcf72
--- /dev/null
+++ b/fuzz/corpus/corpus_001
@@ -0,0 +1,10 @@
+worksheet all_types {
+	1:text      text
+	2:bool      bool
+	3:num_0     number[0]
+	4:num_2     number[2]
+	5:undefined undefined
+	6:ws        all_types
+	7:slice_t   []text
+	8:slice_ws  []all_types
+}
\ No newline at end of file
diff --git a/fuzz/corpus/corpus_002 b/fuzz/corpus/corpus_002
new file mode 100644
index 0000000..44a4812
--- /dev/null
+++ b/fuzz/corpus/corpus_002
@@ -0,0 +1,5 @@
+worksheet a_b_c_d {
+	789 : foo computed_by {
+		external
+	}
+}
\ No newline at end of file
diff --git a/fuzz/fuzz.go b/fuzz/fuzz.go
new file mode 100644
index 0000000..5ea176a
--- /dev/null
+++ b/fuzz/fuzz.go
@@ -0,0 +1,29 @@
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build gofuzz
+
+package worksheets
+
+import (
+	"strings"
+
+	"github.com/helloeave/worksheets"
+)
+
+func Fuzz(data []byte) int {
+	_, err := worksheets.NewDefinitions(strings.NewReader(string(data)))
+	if err != nil {
+		return 1
+	}
+	return 0
+}
diff --git a/parser.go b/parser.go
index 5a474be..af60ea0 100644
--- a/parser.go
+++ b/parser.go
@@ -158,10 +158,13 @@ func (p *parser) parseField() (*Field, error) {
 	if err != nil {
 		return nil, err
 	}
-	index, err := strconv.Atoi(sIndex)
-	if err != nil {
-		// unexpected since sIndex should conform to pIndex
-		panic(err)
+	index := maxFieldIndex + 1
+	if len(sIndex) <= len(strconv.Itoa(maxFieldIndex)) {
+		index, err = strconv.Atoi(sIndex)
+		if err != nil {
+			// unexpected since sIndex should conform to pIndex
+			panic(err)
+		}
 	}
 
 	_, err = p.nextAndCheck(pColon)
@@ -519,17 +522,12 @@ func (p *parser) parseRound() (*tRound, error) {
 	}
 	p.next()
 
-	sIndex, err := p.nextAndCheck(pIndex)
+	scale, err := p.parseScale()
 	if err != nil {
 		return nil, err
 	}
-	index, err := strconv.Atoi(sIndex)
-	if err != nil {
-		// unexpected since sIndex should conform to pIndex
-		panic(err)
-	}
 
-	return &tRound{RoundingMode(mode), index}, nil
+	return &tRound{RoundingMode(mode), scale}, nil
 }
 
 func (p *parser) parseType() (Type, error) {
@@ -563,15 +561,10 @@ func (p *parser) parseType() (Type, error) {
 			if err != nil {
 				return nil, err
 			}
-			sScale, err := p.nextAndCheck(pIndex)
+			scale, err := p.parseScale()
 			if err != nil {
 				return nil, err
 			}
-			scale, err := strconv.Atoi(sScale)
-			if err != nil {
-				// unexpected since sIndex should conform to pIndex
-				panic(err)
-			}
 			_, err = p.nextAndCheck(pRbracket)
 			if err != nil {
 				return nil, err
@@ -600,6 +593,27 @@ func (p *parser) parseType() (Type, error) {
 	}
 }
 
+const maxScale = 32
+
+func (p *parser) parseScale() (int, error) {
+	sScale, err := p.nextAndCheck(pIndex)
+	if err != nil {
+		return -1, err
+	}
+	var scale int = maxScale + 1
+	if len(sScale) <= len(strconv.Itoa(maxScale)) {
+		scale, err = strconv.Atoi(sScale)
+		if err != nil {
+			// unexpected since sScale should conform to pIndex
+			panic(err)
+		}
+	}
+	if scale > maxScale {
+		return -1, fmt.Errorf("scale cannot be greater than 32")
+	}
+	return scale, nil
+}
+
 func (p *parser) parseLiteral() (Value, error) {
 	var err error
 	var negNumber bool
diff --git a/parser_test.go b/parser_test.go
index 86aa614..050cf57 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -241,6 +241,9 @@ func (s *Zuite) TestParser_parseExpressionErrors() {
 		`1_234._67`: `number fraction cannot start with underscore`,
 		`1_234.+7`:  `number cannot terminate with dot`,
 
+		`5 round down 33`:                                                `scale cannot be greater than 32`,
+		`5 round down 9999999999999999999999999999999999999999999999999`: `scale cannot be greater than 32`,
+
 		// will need to revisit when we implement mod operator
 		`4%0`:     `number must terminate with percent if present`,
 		`-1%_000`: `number must terminate with percent if present`,
@@ -304,12 +307,14 @@ func (s *Zuite) TestParser_parseLiteral() {
 
 func (s *Zuite) TestParser_parseType() {
 	cases := map[string]Type{
-		`undefined`: &UndefinedType{},
-		`text`:      &TextType{},
-		`bool`:      &BoolType{},
-		`number[5]`: &NumberType{5},
-		`[]bool`:    &SliceType{&BoolType{}},
-		`foobar`:    &Definition{name: "foobar"},
+		`undefined`:     &UndefinedType{},
+		`text`:          &TextType{},
+		`bool`:          &BoolType{},
+		`number[5]`:     &NumberType{5},
+		`number[32]`:    &NumberType{32},
+		`[]bool`:        &SliceType{&BoolType{}},
+		`[][]number[9]`: &SliceType{&SliceType{&NumberType{9}}},
+		`foobar`:        &Definition{name: "foobar"},
 	}
 	for input, expected := range cases {
 		p := newParser(strings.NewReader(input))
@@ -319,6 +324,19 @@ func (s *Zuite) TestParser_parseType() {
 	}
 }
 
+func (s *Zuite) TestParser_parseTypeErrors() {
+	cases := map[string]string{
+		`number[-7]`: `expected index, found -`,
+		`number[33]`: `scale cannot be greater than 32`,
+		`number[9999999999999999999999999999999999999999999999999]`: `scale cannot be greater than 32`,
+	}
+	for input, expected := range cases {
+		p := newParser(strings.NewReader(input))
+		_, err := p.parseType()
+		assert.EqualError(s.T(), err, expected, input)
+	}
+}
+
 func (s *Zuite) TestTokenPatterns() {
 	cases := []struct {
 		pattern *tokenPattern
diff --git a/tree.go b/tree.go
index 6aab160..c136a45 100644
--- a/tree.go
+++ b/tree.go
@@ -14,8 +14,11 @@ package worksheets
 
 import (
 	"fmt"
+	"math"
 )
 
+const maxFieldIndex = math.MaxUint16
+
 type Definition struct {
 	name          string
 	fieldsByName  map[string]*Field
@@ -32,6 +35,8 @@ func (def *Definition) addField(field *Field) error {
 
 	if _, ok := def.fieldsByIndex[field.index]; ok {
 		return fmt.Errorf("%s.%s: index %d cannot be reused", def.name, field.name, field.index)
+	} else if field.index > maxFieldIndex {
+		return fmt.Errorf("%s.%s: index cannot be greater than %d", def.name, field.name, maxFieldIndex)
 	}
 	def.fieldsByIndex[field.index] = field
 
diff --git a/worksheets_test.go b/worksheets_test.go
index 22f8226..72ada8a 100644
--- a/worksheets_test.go
+++ b/worksheets_test.go
@@ -49,6 +49,18 @@ func (s *Zuite) TestExample() {
 	require.False(s.T(), isSet)
 }
 
+func (s *Zuite) TestNewDefinitionsGood() {
+	cases := []string{
+		`worksheet simple {
+			65535:index_at_max bool
+		}`,
+	}
+	for _, ex := range cases {
+		_, err := NewDefinitions(strings.NewReader(ex))
+		assert.NoError(s.T(), err, ex)
+	}
+}
+
 func (s *Zuite) TestNewDefinitionsErrors() {
 	cases := map[string]string{
 		// crap input
@@ -59,6 +71,14 @@ func (s *Zuite) TestNewDefinitionsErrors() {
 		`work sheet`:      `expecting worksheet`,
 
 		// worksheet semantics
+		`worksheet simple {
+			65536:index_too_large bool
+		}`: `simple.index_too_large: index cannot be greater than 65535`,
+
+		`worksheet simple {
+			9999999999999999999999999999999999999999999999999:index_too_large bool
+		}`: `simple.index_too_large: index cannot be greater than 65535`,
+
 		`worksheet simple {
 			0:no_can_do_with_zero bool
 		}`: `simple.no_can_do_with_zero: index cannot be zero`,