fixes #4307 BigNumber casting issue

ui/app/conversion-util.js

@@ -140,7 +140,7 @@ const addCurrencies = (a, b, options = {}) => {
     bBase,
     ...conversionOptions
   } = options
-  const value = (new BigNumber(a, aBase)).add(b, bBase)
+  const value = (new BigNumber(a.toString(), aBase)).add(b.toString(), bBase)
 
   return converter({
     value,

ui/app/conversion-util.test.js

@@ -0,0 +1,23 @@
+import assert from 'assert'
+import {addCurrencies} from './conversion-util'
+
+
+
+describe('conversion utils', () => {
+  describe.only('addCurrencies()', () => {
+    it('add whole numbers', () => {
+      const result = addCurrencies(3, 5)
+      assert.equal(result.toNumber(), 8)
+    })
+
+    it('add decimals', () => {
+      const result = addCurrencies(1.3, 1.5)
+      assert.equal(result.toNumber(), 2.8)
+    })
+
+    it('add repeating decimals', () => {
+      const result = addCurrencies(1/3, 1/7)
+      assert.equal(result.toNumber(), 0.47619047619047616)
+    })
+  })
+})