diff --git a/lib/serializer.js b/lib/serializer.js
index 0bd2bc5c..ea810293 100644
--- a/lib/serializer.js
+++ b/lib/serializer.js
@@ -50,17 +50,14 @@ module.exports = class Serializer {
   }
 
   asNumber (i) {
-    if (typeof i !== 'number') {
-      i = Number(i)
-    }
-    // NaN !== NaN
-    if (i !== i) { // eslint-disable-line no-self-compare
+    const num = Number(i)
+    if (Number.isNaN(num)) {
       throw new Error(`The value "${i}" cannot be converted to a number.`)
+    } else if (!Number.isFinite(num)) {
+      return null
+    } else {
+      return '' + num
     }
-    if (i === Infinity || i === -Infinity) {
-      return 'null'
-    }
-    return '' + i
   }
 
   asBoolean (bool) {
diff --git a/test/fix-604.test.js b/test/fix-604.test.js
new file mode 100644
index 00000000..ce8c3685
--- /dev/null
+++ b/test/fix-604.test.js
@@ -0,0 +1,24 @@
+'use strict'
+
+const t = require('tap')
+const fjs = require('..')
+const schema = {
+  type: 'object',
+  properties: {
+    fullName: { type: 'string' },
+    phone: { type: 'number' }
+  }
+}
+
+const input = {
+  fullName: 'Jone',
+  phone: 'phone'
+}
+
+const render = fjs(schema)
+
+try {
+  render(input)
+} catch (err) {
+  t.equal(err.message, 'The value "phone" cannot be converted to a number.')
+}