diff --git a/echoprometheus/prometheus.go b/echoprometheus/prometheus.go
index cc864c8..1f9ed82 100644
--- a/echoprometheus/prometheus.go
+++ b/echoprometheus/prometheus.go
@@ -11,17 +11,19 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
+	"net/http"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+
 	"github.com/labstack/echo/v4"
 	"github.com/labstack/echo/v4/middleware"
 	"github.com/labstack/gommon/log"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/prometheus/common/expfmt"
-	"io"
-	"net/http"
-	"sort"
-	"strconv"
-	"time"
 )
 
 const (
@@ -273,10 +275,17 @@ func (conf MiddlewareConfig) ToMiddleware() (echo.MiddlewareFunc, error) {
 			values[2] = c.Request().Host
 			values[3] = url
 			for _, cv := range customValuers {
-				values[cv.index] = cv.valueFunc(c, err)
+				values[cv.index] = strings.ToValidUTF8(cv.valueFunc(c, err), "�")
 			}
 
-			requestDuration.WithLabelValues(values...).Observe(elapsed)
+			// The first metric set is done this way to ensure
+			// that the values all contain valid UTF8.  If not, it
+			// returns, if they are then .WithLabelValues is fine.
+			if obs, tmpErr := requestDuration.GetMetricWithLabelValues(values...); tmpErr == nil {
+				obs.Observe(elapsed)
+			} else {
+				return fmt.Errorf("requestDuration observation failed, err: %w", tmpErr)
+			}
 			requestCount.WithLabelValues(values...).Inc()
 			requestSize.WithLabelValues(values...).Observe(float64(reqSz))
 			responseSize.WithLabelValues(values...).Observe(float64(c.Response().Size))