diff --git a/lib/cat.js b/lib/cat.js
index d8211a6..888a136 100644
--- a/lib/cat.js
+++ b/lib/cat.js
@@ -37,13 +37,14 @@ function cat(...predicates) {
         gen: () => tcg.array(pairs.map(([, p]) => gen(p))),
         describe: () => [cat.name, ...describe(predicates)],
         explain: function*(values, {via}) {
-            yield* explainLength(values, pairs, via);
+            yield* explainInsufficientInput(values, pairs, via);
+            yield* explainExtraInput(pairs, values, via);
             yield* explainInvalid(values, pairs, via);
         }
     };
 }
 
-function* explainLength(values, specs, via) {
+function* explainInsufficientInput(values, specs, via) {
     // TODO: not correct for complicated cases. Use _conform
     if (values.length < specs.length) {
         const [key] = specs[values.length];
@@ -58,6 +59,19 @@ function* explainLength(values, specs, via) {
     }
 }
 
+function* explainExtraInput(specs, values, via) {
+    if (values.length > specs.length) {
+        yield {
+            path: [],
+            reason: 'Extra input',
+            pred: ['cat', ...describe(specs)],
+            val: _.tail(values),
+            via,
+            'in': [1]
+        };
+    }
+}
+
 function* explainInvalid(values, specs, via) {
     if (values.length !== specs.length) return;
 
diff --git a/test/cat.js b/test/cat.js
index 996455e..fe0e4c3 100644
--- a/test/cat.js
+++ b/test/cat.js
@@ -64,7 +64,22 @@ describe('Test the cat function', () => {
                 ]
             });
         });
-        
+
+        xit('explainData should reject extra input', () => {
+            expect(s.explainData('::ingredient', [2, ':teaspoon', 13])).to.be.eql({
+                problems: [
+                    {
+                        path: [],
+                        reason: 'Extra input',
+                        pred: ['cat', ':quantity', 'isNumber', ':unit', 'isString'],
+                        val: [true],
+                        via: ['::bool-or-string'],
+                        'in': [1]
+                    }
+                ]
+            });
+        });
+
         it('explainData should report about wrong type', () => {
             expect(s.explainData('::ingredient', [2, 13])).to.eql({
                 problems: [