diff --git a/watermark.go b/watermark.go
index d5326ef..dd14464 100644
--- a/watermark.go
+++ b/watermark.go
@@ -74,6 +74,9 @@ func (w *WatermarkOption) do(base image.Image) image.Image {
 }
 
 func randRange(min, max int) int {
+	if max < min {
+		min, max = max, min
+	}
 	return rand.Intn(max-min+1) + min
 }
 
diff --git a/watermark_test.go b/watermark_test.go
index feb3d30..8fa1204 100644
--- a/watermark_test.go
+++ b/watermark_test.go
@@ -3,6 +3,7 @@ package imgconv
 import (
 	"image"
 	"reflect"
+	"slices"
 	"testing"
 	"time"
 
@@ -60,3 +61,25 @@ func TestCalcResizeXY(t *testing.T) {
 		}
 	}
 }
+
+func TestRandRange(t *testing.T) {
+	testCase := []struct {
+		min, max int
+		res      []int
+	}{
+		{1, 5, []int{1, 2, 3, 4, 5}},
+		{5, 1, []int{1, 2, 3, 4, 5}},
+		{-1, 5, []int{-1, 0, 1, 2, 3, 4, 5}},
+		{5, -1, []int{-1, 0, 1, 2, 3, 4, 5}},
+		{-5, -1, []int{-5, -4, -3, -2, -1}},
+		{-1, -5, []int{-5, -4, -3, -2, -1}},
+	}
+
+	for i := 0; i < 100; i++ {
+		for i, tc := range testCase {
+			if res := randRange(tc.min, tc.max); !slices.Contains(tc.res, res) {
+				t.Errorf("#%d: got %d, not in range %v", i, res, tc.res)
+			}
+		}
+	}
+}