diff --git a/setup/config/config_appservice.go b/setup/config/config_appservice.go
index 4a14672e6..9bd8a1b51 100644
--- a/setup/config/config_appservice.go
+++ b/setup/config/config_appservice.go
@@ -101,7 +101,7 @@ func (a *ApplicationService) IsInterestedInRoomID(
 ) bool {
 	if namespaceSlice, ok := a.NamespaceMap["rooms"]; ok {
 		for _, namespace := range namespaceSlice {
-			if namespace.RegexpObject.MatchString(roomID) {
+			if namespace.RegexpObject != nil && namespace.RegexpObject.MatchString(roomID) {
 				return true
 			}
 		}
@@ -222,6 +222,10 @@ func setupRegexps(asAPI *AppServiceAPI, derived *Derived) (err error) {
 			case "aliases":
 				appendExclusiveNamespaceRegexs(&exclusiveAliasStrings, namespaceSlice)
 			}
+
+			if err = compileNamespaceRegexes(namespaceSlice); err != nil {
+				return fmt.Errorf("invalid regex in appservice %q, namespace %q: %w", appservice.ID, key, err)
+			}
 		}
 	}
 
@@ -258,18 +262,31 @@ func setupRegexps(asAPI *AppServiceAPI, derived *Derived) (err error) {
 func appendExclusiveNamespaceRegexs(
 	exclusiveStrings *[]string, namespaces []ApplicationServiceNamespace,
 ) {
-	for index, namespace := range namespaces {
+	for _, namespace := range namespaces {
 		if namespace.Exclusive {
 			// We append parenthesis to later separate each regex when we compile
 			// i.e. "app1.*", "app2.*" -> "(app1.*)|(app2.*)"
 			*exclusiveStrings = append(*exclusiveStrings, "("+namespace.Regex+")")
 		}
-
-		// Compile this regex into a Regexp object for later use
-		namespaces[index].RegexpObject, _ = regexp.Compile(namespace.Regex)
 	}
 }
 
+// compileNamespaceRegexes turns strings into regex objects and complains
+// if some of there are bad
+func compileNamespaceRegexes(namespaces []ApplicationServiceNamespace) (err error) {
+	for index, namespace := range namespaces {
+		// Compile this regex into a Regexp object for later use
+		r, err := regexp.Compile(namespace.Regex)
+		if err != nil {
+			return fmt.Errorf("regex at namespace %d: %w", index, err)
+		}
+
+		namespaces[index].RegexpObject = r
+	}
+
+	return nil
+}
+
 // checkErrors checks for any configuration errors amongst the loaded
 // application services according to the application service spec.
 func checkErrors(config *AppServiceAPI, derived *Derived) (err error) {