diff --git a/numbers.go b/numbers.go
index 551844e..55483fd 100644
--- a/numbers.go
+++ b/numbers.go
@@ -165,10 +165,15 @@ func formatIntWithSep(n int64, sep string) string {
 		return strconv.FormatInt(n, 10)
 	}
 	negative := n < 0
+	var abs uint64
 	if negative {
-		n = -n
+		// Convert via n+1 to avoid overflowing on math.MinInt64.
+		abs = uint64(-(n + 1))
+		abs++
+	} else {
+		abs = uint64(n)
 	}
-	str := strconv.FormatInt(n, 10)
+	str := strconv.FormatUint(abs, 10)
 	if len(str) <= 3 {
 		if negative {
 			return "-" + str
diff --git a/numbers_test.go b/numbers_test.go
index 1b20313..33ebfd3 100644
--- a/numbers_test.go
+++ b/numbers_test.go
@@ -1,6 +1,9 @@
 package i18n
 
-import "testing"
+import (
+	"math"
+	"testing"
+)
 
 func TestFormatNumber(t *testing.T) {
 	// Ensure service is initialised for English locale
@@ -31,6 +34,20 @@ func TestFormatNumber(t *testing.T) {
 	}
 }
 
+func TestFormatNumber_MinInt64(t *testing.T) {
+	svc, err := New()
+	if err != nil {
+		t.Fatalf("New() failed: %v", err)
+	}
+	SetDefault(svc)
+
+	got := FormatNumber(math.MinInt64)
+	want := "-9,223,372,036,854,775,808"
+	if got != want {
+		t.Fatalf("FormatNumber(math.MinInt64) = %q, want %q", got, want)
+	}
+}
+
 func TestFormatDecimal(t *testing.T) {
 	svc, err := New()
 	if err != nil {