diff --git a/command/agent/stats_endpoint.go b/command/agent/stats_endpoint.go
index 5dabdf9f9..0cf5b8dc5 100644
--- a/command/agent/stats_endpoint.go
+++ b/command/agent/stats_endpoint.go
@@ -6,6 +6,10 @@ import (
 	"strconv"
 )
 
+const (
+	invalidSinceErrPrefix = "can't read the since query parameter"
+)
+
 func (s *HTTPServer) ClientStatsRequest(resp http.ResponseWriter, req *http.Request) (interface{}, error) {
 	if s.agent.client == nil {
 		return nil, clientNotRunning
@@ -18,7 +22,7 @@ func (s *HTTPServer) ClientStatsRequest(resp http.ResponseWriter, req *http.Requ
 		ts = true
 		since, err = strconv.Atoi(sinceTime)
 		if err != nil {
-			return nil, CodedError(400, fmt.Sprintf("can't read the since query parameter: %v", err))
+			return nil, CodedError(400, fmt.Sprintf("%s: %v", invalidSinceErrPrefix, err))
 		}
 	}
 
diff --git a/command/agent/stats_endpoint_test.go b/command/agent/stats_endpoint_test.go
new file mode 100644
index 000000000..6f9649ab6
--- /dev/null
+++ b/command/agent/stats_endpoint_test.go
@@ -0,0 +1,23 @@
+package agent
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestClientStatsRequest(t *testing.T) {
+	httpTest(t, nil, func(s *TestServer) {
+		req, err := http.NewRequest("GET", "/v1/client/stats/?since=foo", nil)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+
+		respW := httptest.NewRecorder()
+		_, err = s.Server.ClientStatsRequest(respW, req)
+		if !strings.ContainsAny(err.Error(), invalidSinceErrPrefix) {
+			t.Fatalf("unexpected err: %v", err)
+		}
+	})
+}