diff --git a/converters.go b/converters.go
index ef86d95b..f73b9356 100644
--- a/converters.go
+++ b/converters.go
@@ -32,7 +32,9 @@ func (c *converters) parseAlphaField(r string, max uint) string {
 	if ln > max {
 		return r[ln-max:]
 	}
-	r += strings.Repeat(" ", int(max-ln))
+	if count := int(max - ln); validSize(count) {
+		r += strings.Repeat(" ", count)
+	}
 	return r
 }
 
@@ -42,7 +44,9 @@ func (c *converters) numericStringField(s string, max uint) string {
 	if ln > max {
 		return s[ln-max:]
 	}
-	s = strings.Repeat("0", int(max-ln)) + s
+	if count := int(max - ln); validSize(count) {
+		s = strings.Repeat("0", count) + s
+	}
 	return s
 }
 
@@ -60,11 +64,11 @@ func (c *converters) formatAlphaField(s string, max uint, options FormatOptions)
 	if ln > max {
 		return s[:max]
 	}
-
 	if !options.VariableLengthFields {
-		s += strings.Repeat(" ", int(max-ln))
+		if count := int(max - ln); validSize(count) {
+			s += strings.Repeat(" ", count)
+		}
 	}
-
 	return s
 }
 
diff --git a/unstructuredAddenda.go b/unstructuredAddenda.go
index 38ec2566..2243037e 100644
--- a/unstructuredAddenda.go
+++ b/unstructuredAddenda.go
@@ -73,7 +73,11 @@ func (ua *UnstructuredAddenda) String() string {
 	buf.Grow(10)
 	buf.WriteString(ua.tag)
 	buf.WriteString(ua.AddendaLengthField())
-	buf.Grow(ua.parseNumField(ua.AddendaLength))
+
+	if size := ua.parseNumField(ua.AddendaLength); validSize(size) {
+		buf.Grow(size)
+	}
+
 	buf.WriteString(ua.AddendaField())
 	return buf.String()
 }
diff --git a/validators.go b/validators.go
index 82c8f9b4..297eb5d0 100644
--- a/validators.go
+++ b/validators.go
@@ -24,6 +24,17 @@ var (
 	amountRegex  = regexp.MustCompile("[^0-9,.]")
 )
 
+const (
+	// maxBufferGrowth is the high limit for growing string builders and byte buffers.
+	//
+	// 1e8/1024/1024 is ~95MB which should be enough for anybody
+	maxBufferGrowth = 1e8
+)
+
+func validSize(n int) bool {
+	return n > 0 && n < maxBufferGrowth
+}
+
 // validator is common validation and formatting of golang types to WIRE type strings
 type validator struct{}
 
diff --git a/validators_test.go b/validators_test.go
index bf6c8bfa..e75e4db1 100644
--- a/validators_test.go
+++ b/validators_test.go
@@ -5,11 +5,29 @@
 package wire
 
 import (
+	"fmt"
+	"math"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
+func TestValidSize(t *testing.T) {
+	require.True(t, validSize(10))
+	require.True(t, validSize(1e7))
+
+	require.False(t, validSize(1e8+1))
+	require.False(t, validSize(1e9))
+	require.False(t, validSize(math.MaxInt))
+
+	t.Run("don't grow", func(t *testing.T) {
+		ua := &UnstructuredAddenda{}
+		ua.AddendaLength = fmt.Sprintf("%0.0f", 1e9)
+		expected := "1000"
+		require.Equal(t, expected, ua.String())
+	})
+}
+
 func TestValidators__validateOptionFName(t *testing.T) {
 	v := &validator{}