diff --git a/lib/plus.js b/lib/plus.js
index deb2e5e..f0d1191 100644
--- a/lib/plus.js
+++ b/lib/plus.js
@@ -18,40 +18,43 @@ function plus(spec) {
         gen: () => tcg.null.then(() => tcg.array(gen(spec), {size: _.random(1, 5)})),
         describe: () => [plus.name, ...describe([spec])],
         explain: (values, {via}) => {
-            if (values.length === 0) {
-                return {
-                    problems: [
-                        {
-                            path: [],
-                            reason: 'Insufficient input',
-                            val: values,
-                            via,
-                            'in': []
-                        }
-                    ]
-                };
-            } else {
-                const index = _.findIndex(values, value => !isValid(spec, value));
-                if (index === -1) {
-                    return null;
-                } else {
-                    const val = values[index];
-                    const pred = explainData(spec, val).problems[0].pred;
-                    return {
-                        problems: [
-                            {
-                                path: [],
-                                pred,
-                                val,
-                                via,
-                                'in': [index]
-                            }
-                        ]
-                    };
-                }
+            const problems = [];
+            explainEmptyInput(problems, values, via);
+            if (_.isEmpty(problems)) {
+                explainValidInput(problems, spec, values, via);
             }
+            return _.isEmpty(problems) ? null : {problems};
         }
     };
 }
 
+function explainEmptyInput(problems, values, via) {
+    if (values.length === 0) {
+        problems.push({
+            path: [],
+            reason: 'Insufficient input',
+            val: values,
+            via,
+            'in': []
+        });
+    }
+    return problems;
+}
+
+function explainValidInput(problems, spec, values, via) {
+    const index = _.findIndex(values, value => !isValid(spec, value));
+    if (index !== -1) {
+        const val = values[index];
+        const pred = explainData(spec, val).problems[0].pred;
+        problems.push({
+            path: [],
+            pred,
+            val,
+            via,
+            'in': [index]
+        });    
+    }
+    return problems;
+}
+
 module.exports = plus;
diff --git a/test/plus.js b/test/plus.js
index f11a7a8..d96f4e2 100644
--- a/test/plus.js
+++ b/test/plus.js
@@ -13,51 +13,61 @@ describe('Test the plus (+) function', () => {
         s.def('::odd?', s.and(isInteger, isOdd));
         s.def('::odds', s.plus('::odd?'))
     });
-    
-    it('should return the value', () => {
-        expect(s.conform('::odds', [1, 3])).to.eql([1, 3]);
-    });
-    
-    it('should return the invalid string when value sequence is empty', () => {
-        expect(s.conform('::odds', [])).to.equal(invalidString);
-    });
-    
-    it('should return the invalid string', () => {
-        expect(s.conform('::odds', [1, 3, 6])).to.equal(invalidString);
-    });
 
-    it('should unform a conformed value', () => {
-        expect(idemPotent('::odds', [1, 3, 5])).to.be.true;
+    describe('should handle valid input', () => {
+        it('conform should return the value', () => {
+            expect(s.conform('::odds', [1, 3])).to.eql([1, 3]);
+        });
+
+        it('explainData should return null', () => {
+            expect(s.explainData('::odds', [1, 3])).to.be.null;
+        });
     });
 
-    it('should implement explain', () => {
-        expect(s.explainData('::odds', [1, 3, 5])).to.be.null;
+    describe('should reject empty input', () => {
+        it('conform should return the invalid string when value sequence is empty', () => {
+            expect(s.conform('::odds', [])).to.equal(invalidString);
+        });
 
-        expect(s.explainData('::odds', [])).to.eql({
-            problems: [
-                {
-                    path: [],
-                    reason: 'Insufficient input',
-                    val: [],
-                    via: ['::odds'],
-                    'in': []
-                }
-            ]
+        it('explainData should report a problem', () => {
+            expect(s.explainData('::odds', [])).to.eql({
+                problems: [
+                    {
+                        path: [],
+                        reason: 'Insufficient input',
+                        val: [],
+                        via: ['::odds'],
+                        'in': []
+                    }
+                ]
+            });
+        });
+    });
+    
+    describe('should reject invalid input', () => {
+        it('should return the invalid string', () => {
+            expect(s.conform('::odds', [1, 3, 6])).to.equal(invalidString);
         });
 
-        expect(s.explainData('::odds', [1, 3, 6])).to.eql({
-            problems: [
-                {
-                    path: [],
-                    pred: 'isOdd',
-                    val: 6,
-                    via: ['::odds'],
+        it('explainData should report a problem', () => {
+            expect(s.explainData('::odds', [1, 3, 6])).to.eql({
+                problems: [
+                    {
+                        path: [],
+                        pred: 'isOdd',
+                        val: 6,
+                        via: ['::odds'],
                     'in': [2]
-                }
-            ]
+                    }
+                ]
+            });
         });
     });
 
+    it('should unform a conformed value', () => {
+        expect(idemPotent('::odds', [1, 3, 5])).to.be.true;
+    });
+
     it('should implement a generator', () => {
         expect(s.exercise(s.plus(isInteger))).to.have.length(10)
             .to.satisfy(sample => _.every(sample, ([v]) => _.isArray(v) && v.length > 0));