diff --git a/builtin/builtin_test.go b/builtin/builtin_test.go
index aa324c9b..7f5045f4 100644
--- a/builtin/builtin_test.go
+++ b/builtin/builtin_test.go
@@ -612,3 +612,17 @@ func TestBuiltin_bitOpsFunc(t *testing.T) {
 		})
 	}
 }
+
+type customInt int
+
+func Test_int_unwraps_underlying_value(t *testing.T) {
+	env := map[string]any{
+		"customInt": customInt(42),
+	}
+	program, err := expr.Compile(`int(customInt) == 42`, expr.Env(env))
+	require.NoError(t, err)
+
+	out, err := expr.Run(program, env)
+	require.NoError(t, err)
+	assert.Equal(t, true, out)
+}
diff --git a/builtin/lib.go b/builtin/lib.go
index 9ff9478a..e3a6c0ae 100644
--- a/builtin/lib.go
+++ b/builtin/lib.go
@@ -209,6 +209,10 @@ func Int(x any) any {
 		}
 		return i
 	default:
+		val := reflect.ValueOf(x)
+		if val.CanConvert(integerType) {
+			return val.Convert(integerType).Interface()
+		}
 		panic(fmt.Sprintf("invalid operation: int(%T)", x))
 	}
 }