diff --git a/run.py b/run.py
index eebda61..cc5d8ee 100755
--- a/run.py
+++ b/run.py
@@ -4,7 +4,7 @@
 import os
 import yaml
 import base64
-from flask import Flask, render_template, request
+from flask import Flask, render_template, request, Response
 
 from sigma.conversion.base import Backend
 from sigma.plugins import InstalledSigmaPlugins
@@ -15,45 +15,53 @@
 plugins = InstalledSigmaPlugins.autodiscover()
 backends = plugins.backends
 pipeline_resolver = plugins.get_pipeline_resolver()
-pipelines = list(pipeline_resolver.list_pipelines()) 
+pipelines = list(pipeline_resolver.list_pipelines())
 
-@app.route('/')
+
+@app.route("/")
 def home():
     formats = []
     for backend in backends.keys():
         for name, description in plugins.backends[backend].formats.items():
-            formats.append({"name": name, "description": description, "backend": backend})
-    
+            formats.append(
+                {"name": name, "description": description, "backend": backend}
+            )
+
     for name, pipeline in pipelines:
         if len(pipeline.allowed_backends) > 0:
             pipeline.backends = ", ".join(pipeline.allowed_backends)
         else:
             pipeline.backends = "all"
 
-    return render_template('index.html', backends=backends, pipelines=pipelines, formats=formats)
+    return render_template(
+        "index.html", backends=backends, pipelines=pipelines, formats=formats
+    )
 
-@app.route('/sigma', methods=['POST'])
-def convert():
 
+@app.route("/sigma", methods=["POST"])
+def convert():
     # get params from request
-    rule = str(base64.b64decode(request.json['rule']), "utf-8")
+    rule = str(base64.b64decode(request.json["rule"]), "utf-8")
     # check if input is valid yaml
     try:
         yaml.safe_load(rule)
     except:
         print("error")
-        return ("Error: No valid yaml input")
+        return Response(
+            f"YamlError: Malformed yaml file", status=400, mimetype="text/html"
+        )
 
     pipeline = []
-    if request.json['pipeline']:
-        for p in request.json['pipeline']:
+    if request.json["pipeline"]:
+        for p in request.json["pipeline"]:
             pipeline.append(p)
-    target = request.json['target']
-    format = request.json['format']
+
+    target = request.json["target"]
+    format = request.json["format"]
 
     backend_class = backends[target]
     processing_pipeline = pipeline_resolver.resolve(pipeline)
-    backend : Backend = backend_class(processing_pipeline=processing_pipeline)
+    backend: Backend = backend_class(processing_pipeline=processing_pipeline)
 
     try:
         sigma_rule = SigmaCollection.from_yaml(rule)
@@ -61,9 +69,10 @@ def convert():
         if isinstance(result, list):
             result = result[0]
     except SigmaError as e:
-        return "Error: " + str(e)
+        return Response(f"SigmaError: {str(e)}", status=400, mimetype="text/html")
 
     return result
 
-if __name__ == '__main__':
-    app.run(host='0.0.0.0', port=int(os.environ.get('PORT', 8000)))
+
+if __name__ == "__main__":
+    app.run(host="0.0.0.0", port=int(os.environ.get("PORT", 8000)))
diff --git a/static/js/index.js b/static/js/index.js
index cef73db..f53c595 100644
--- a/static/js/index.js
+++ b/static/js/index.js
@@ -127,6 +127,8 @@ function convert(sigmaRule) {
         queryCode.innerHTML = xhr.response;
         queryCode.value = xhr.response;
         Prism.highlightElement(queryCode); // rerun code highlighting
+      } else if (xhr.status === 400) {
+        queryCode.innerHTML = xhr.response;
       } else if (xhr.status === 500) {
         queryCode.innerHTML = "Error: Something went wrong";
       }