diff --git a/cmd/prometheus/config.go b/cmd/prometheus/config.go
index 73f17986d2..2460e96f7a 100644
--- a/cmd/prometheus/config.go
+++ b/cmd/prometheus/config.go
@@ -319,6 +319,11 @@ func parse(args []string) error {
 	return nil
 }
 
+func startsOrEndsWithQuote(s string) bool {
+	return strings.HasPrefix(s, "\"") || strings.HasPrefix(s, "'") ||
+		strings.HasSuffix(s, "\"") || strings.HasSuffix(s, "'")
+}
+
 func parsePrometheusURL() error {
 	if cfg.prometheusURL == "" {
 		hostname, err := os.Hostname()
@@ -330,6 +335,8 @@ func parsePrometheusURL() error {
 			return err
 		}
 		cfg.prometheusURL = fmt.Sprintf("http://%s:%s/", hostname, port)
+	} else if startsOrEndsWithQuote(cfg.prometheusURL) {
+		return fmt.Errorf("web.external-url must not begin or end with quotes")
 	}
 
 	promURL, err := url.Parse(cfg.prometheusURL)
@@ -349,6 +356,8 @@ func parsePrometheusURL() error {
 func validateAlertmanagerURL(u string) error {
 	if u == "" {
 		return nil
+	} else if startsOrEndsWithQuote(u) {
+		return fmt.Errorf("alertmanager.url must not begin or end with quotes: %s", u)
 	}
 	url, err := url.Parse(u)
 	if err != nil {
diff --git a/cmd/prometheus/config_test.go b/cmd/prometheus/config_test.go
index f26f8cbe1d..3248820d07 100644
--- a/cmd/prometheus/config_test.go
+++ b/cmd/prometheus/config_test.go
@@ -40,6 +40,10 @@ func TestParse(t *testing.T) {
 			input: []string{"-web.external-url", "'https://url/prometheus'"},
 			valid: false,
 		},
+		{
+			input: []string{"-web.external-url", "'relative/path/with/quotes'"},
+			valid: false,
+		},
 		{
 			input: []string{"-alertmanager.url", ""},
 			valid: true,
@@ -56,6 +60,14 @@ func TestParse(t *testing.T) {
 			input: []string{"-alertmanager.url", "https://double--dash.de"},
 			valid: true,
 		},
+		{
+			input: []string{"-alertmanager.url", "'http://starts/with/quote"},
+			valid: false,
+		},
+		{
+			input: []string{"-alertmanager.url", "ends/with/quote\""},
+			valid: false,
+		},
 	}
 
 	for i, test := range tests {