diff --git a/docs/datatypes/bignumbers.md b/docs/datatypes/bignumbers.md
index 657c96f81..1d53c6ae7 100644
--- a/docs/datatypes/bignumbers.md
+++ b/docs/datatypes/bignumbers.md
@@ -33,9 +33,7 @@ math.eval('0.1 + 0.2'); // BigNumber, 0.3
 The default precision for BigNumber is 64 digits, and can be configured with
 the option `precision`.
 
-*Important:
-BigNumber is not supported by all functions, like `arg`, and `random`.
-These functions will downgrade BigNumber to Number, and return a Number.*
+Note that BigNumbers are not supported by all functions.
 
 
 ## Round-off errors
diff --git a/lib/core/typed.js b/lib/core/typed.js
index 00e9a758c..6acd58455 100644
--- a/lib/core/typed.js
+++ b/lib/core/typed.js
@@ -80,6 +80,13 @@ exports.create = function create(type) {
       convert: function (x) {
         return new type.Complex(x.toNumber(), 0);
       }
+    }, {
+      from: 'Fraction',
+      to: 'BigNumber',
+      convert: function (x) {
+        throw new TypeError('Cannot implicitly convert a Fraction to BigNumber or vice versa. ' +
+            'Use function bignumber(x) to convert to BigNumber or fraction(x) to convert to Fraction.');
+      }
     }, {
       from: 'Fraction',
       to: 'Complex',
diff --git a/test/function/arithmetic/multiply.test.js b/test/function/arithmetic/multiply.test.js
index f64790b4c..0a002485b 100644
--- a/test/function/arithmetic/multiply.test.js
+++ b/test/function/arithmetic/multiply.test.js
@@ -59,6 +59,11 @@ describe('multiply', function() {
       assert.throws(function () {multiply(bignumber(1).div(3), 1/3);}, /Cannot implicitly convert a number with >15 significant digits to BigNumber/);
     });
 
+    it('should throw an error when multipling mixed fractions and bignumbers', function() {
+      assert.throws(function () {multiply(math.bignumber('2'), math.fraction(1,3))}, /Cannot implicitly convert a Fraction to BigNumber/);
+      assert.throws(function () {multiply(math.fraction(1,3), math.bignumber('2'))}, /Cannot implicitly convert a Fraction to BigNumber/);
+    });
+
     it('should multiply mixed booleans and bignumbers', function() {
       assert.deepEqual(multiply(bignumber(0.3), true), bignumber(0.3));
       assert.deepEqual(multiply(bignumber(0.3), false), bignumber(0));