diff --git a/src/sparse_array/core.clj b/src/sparse_array/core.clj
index 6906a36..be8a3f0 100644
--- a/src/sparse_array/core.clj
+++ b/src/sparse_array/core.clj
@@ -32,17 +32,18 @@
   ([x & axes]
    (and
      (map? x)
+     (number? (:dimensions x))
      (pos? (:dimensions x))
      (keyword? (:coord x))
      (= (:coord x) (first axes))
      (if
-       (rest axes)
+       (empty? (rest axes))
+       (= (:content x) :data)
        (and
          (= (:content x) (rest axes))
          (every?
            sparse-array?
-           (map #(x %) (filter integer? (keys x)))))
-       (= (:content x) :data)))))
+           (map #(x %) (filter integer? (keys x)))))))))
 
 (defn put
   "Return a sparse array like this `array` but with this `value` at these
diff --git a/test/sparse_array/core_test.clj b/test/sparse_array/core_test.clj
index 216681a..b7d5707 100644
--- a/test/sparse_array/core_test.clj
+++ b/test/sparse_array/core_test.clj
@@ -16,8 +16,38 @@
                            :coord :y,
                            :content :data,
                            3 "goodbye"}}))
-    (is-not (sparse-array? []))
-    (is-not (sparse-array? "hello"))
+    (is (= (sparse-array? []) false))
+    (is (= (sparse-array? "hello") false))
+    (is (=
+          (sparse-array?
+            (dissoc (make-sparse-array :x :y :z) :dimensions))
+          false)
+        "All mandatory keywords must be present: dimensions")
+    (is (=
+          (sparse-array?
+            (dissoc (make-sparse-array :x :y :z) :coord))
+          false)
+        "All mandatory keywords must be present: coord")
+    (is (=
+          (sparse-array?
+            (dissoc (make-sparse-array :x :y :z) :content))
+          false)
+        "All mandatory keywords must be present: content")
+    (is (=
+          (sparse-array? {:dimensions 2,
+                          :coord :x,
+                          :content '(:y),
+                          3 {:dimensions 1,
+                             :coord :y,
+                             :content :data,
+                             4 "hello"},
+                          4 {:dimensions 1,
+                             :coord :y,
+                             :content :data,
+                             3 "goodbye"}
+                          5 :foo})
+          false)
+          "Can't have data in a non-data layer")
     ))
 
 (deftest put-and-get