diff --git a/src/forms.rs b/src/forms.rs
index 2a4f5d7d..35a4a477 100644
--- a/src/forms.rs
+++ b/src/forms.rs
@@ -228,7 +228,13 @@ impl PredicateClause {
                     _ => None,
                 }
             },
-            PredicateClause::Rule(ref rule, ..) => Some(&rule.head.1),
+            PredicateClause::Rule(ref rule, ..) => {
+                if rule.head.1.is_empty() {
+                    None
+                } else {
+                    Some(&rule.head.1)
+                }
+            }
         }
     }