diff --git a/decimal.go b/decimal.go
index 954a747..b05f9aa 100644
--- a/decimal.go
+++ b/decimal.go
@@ -24,7 +24,7 @@ func marshalDecimal(b io.StringWriter, d float64) error {
 	rounded := math.RoundToEven(d/TH) * TH
 	i, _ := math.Modf(rounded)
 
-	if i < -999999999999 || i > 999999999999 {
+	if math.IsNaN(d) || i < -999999999999 || i > 999999999999 {
 		return ErrInvalidDecimal
 	}
 
diff --git a/decimal_test.go b/decimal_test.go
index c079c6d..ea23ab1 100644
--- a/decimal_test.go
+++ b/decimal_test.go
@@ -1,6 +1,7 @@
 package httpsfv
 
 import (
+	"math"
 	"strings"
 	"testing"
 )
@@ -24,6 +25,7 @@ func TestMarshalDecimal(t *testing.T) {
 		{-9999999999999.0, "", false},
 		{9999999999999.0, "", false},
 		{1.9, "1.9", true},
+		{math.NaN(), "", false},
 	}
 
 	var b strings.Builder