diff --git a/server/routes.go b/server/routes.go
index f9749db2..8c95f158 100644
--- a/server/routes.go
+++ b/server/routes.go
@@ -216,7 +216,11 @@ func GenerateHandler(c *gin.Context) {
 	case req.Prompt != "":
 		prompt, err = promptFromRequestParams(c, model, req)
 		if err != nil {
-			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			status := http.StatusInternalServerError
+			if errors.Is(err, errInvalidRole) {
+				status = http.StatusBadRequest
+			}
+			c.JSON(status, gin.H{"error": err.Error()})
 			return
 		}
 		sendContext = true
@@ -325,6 +329,8 @@ func promptFromRequestParams(c *gin.Context, model *Model, req api.GenerateReque
 	return prompt.String(), nil
 }
 
+var errInvalidRole = errors.New("invalid message role")
+
 func promptFromMessages(model *Model, messages []api.Message) (string, error) {
 	flush := func(vars *PromptVars, model *Model, prompt *strings.Builder) error {
 		p, err := model.Prompt(vars)
@@ -360,6 +366,8 @@ func promptFromMessages(model *Model, messages []api.Message) (string, error) {
 			vars.Prompt = m.Content
 		case "assistant":
 			prompt.WriteString(m.Content)
+		default:
+			return "", fmt.Errorf("%w %q, role must be one of [system, user, assistant]", errInvalidRole, m.Role)
 		}
 	}