diff --git a/sdjwt/pex.py b/sdjwt/pex.py
index 1181b2f..64234ae 100644
--- a/sdjwt/pex.py
+++ b/sdjwt/pex.py
@@ -99,6 +99,7 @@ class PresentationDefinition(BaseModel):
                         "fields": {
                             "type": "array",
                             "items": {"$ref": "#/definitions/field"},
+                            "minItems": 1,
                         },
                     },
                     "required": ["fields"],