diff --git a/src/atom/parsers.rs b/src/atom/parsers.rs
index 8da8203..c609a1c 100644
--- a/src/atom/parsers.rs
+++ b/src/atom/parsers.rs
@@ -185,6 +185,12 @@ impl<'a> Parseable<'a, &'a str> for Slot {
                 sub,
                 operator,
             })
+            .verify_output(|slot| {
+                matches!(
+                    (&slot.primary(), &slot.operator()),
+                    (Some(_) | None, Some(_)) | (Some(_), None)
+                )
+            })
     }
 }
 
@@ -420,9 +426,9 @@ mod test {
 
     #[test]
     fn test_empty_slot() {
-        let it = InputIter::new("foo/bar:=");
+        let it = InputIter::new("=dev-ml/uucp-17*:");
 
-        Atom::parser().check_finished(it).unwrap();
+        assert!(Atom::parser().check_finished(it).is_err());
     }
 
     #[test]
@@ -494,4 +500,11 @@ mod test {
 
         assert!(Cpv::parser().parse_finished(it).is_err());
     }
+
+    #[test]
+    fn test_empty_slot_with_operator() {
+        let it = InputIter::new("foo/bar:=");
+
+        Atom::parser().check_finished(it).unwrap();
+    }
 }