diff --git a/zuul/layoutvalidator.py b/zuul/layoutvalidator.py
index 6969653..5a32c76 100644
--- a/zuul/layoutvalidator.py
+++ b/zuul/layoutvalidator.py
@@ -193,6 +193,9 @@
         return parameters
 
     def getSchema(self, data):
+        if not isinstance(data, dict):
+            raise Exception("Malformed layout configuration: top-level type "
+                            "should be a dictionary")
         pipelines = data.get('pipelines')
         if not pipelines:
             pipelines = []