diff --git a/src/parseTree.js b/src/parseTree.js
index 3adba82..44f38c6 100644
--- a/src/parseTree.js
+++ b/src/parseTree.js
@@ -9,6 +9,9 @@ var Parser = require("./Parser");
  * Parses an expression using a Parser, then returns the parsed result.
  */
 var parseTree = function(toParse, settings) {
+    if (!(typeof toParse === 'string' || toParse instanceof String)) {
+        throw new TypeError('KaTeX can only parse string typed expression');
+    }
     var parser = new Parser(toParse, settings);
 
     return parser.parse();
diff --git a/test/katex-spec.js b/test/katex-spec.js
index af9688d..57aad4e 100644
--- a/test/katex-spec.js
+++ b/test/katex-spec.js
@@ -1382,6 +1382,33 @@ describe("An HTML font tree-builder", function() {
         span = "<span class=\"mord mathbb\" style=\"color:blue;\">R</span>";
         expect(markup).toContain(span);
     });
+
+    it("should throw TypeError when the expression is of the wrong type", function() {
+        expect(function() {
+            katex.renderToString({badInputType: "yes"});
+        }).toThrowError(TypeError);
+        expect(function() {
+            katex.renderToString([1, 2]);
+        }).toThrowError(TypeError);
+        expect(function() {
+            katex.renderToString(undefined);
+        }).toThrowError(TypeError);
+        expect(function() {
+            katex.renderToString(null);
+        }).toThrowError(TypeError);
+        expect(function() {
+            katex.renderToString(1.234);
+        }).toThrowError(TypeError);
+    });
+
+    it("should not throw TypeError when the expression is a supported type", function() {
+        expect(function() {
+            katex.renderToString("\\sqrt{123}");
+        }).not.toThrowError(TypeError);
+        expect(function() {
+            katex.renderToString(new String("\\sqrt{123}"));
+        }).not.toThrowError(TypeError);
+    });
 });
 
 
@@ -1878,3 +1905,15 @@ describe("A macro expander", function() {
         });
     });
 });
+
+describe("A parser taking String objects", function() {
+    it("should not fail on an empty String object", function() {
+        expect(new String("")).toParse();
+    });
+
+    it("should parse the same as a regular string", function() {
+        expect(new String("xy")).toParseLike("xy");
+        expect(new String("\\div")).toParseLike("\\div");
+        expect(new String("\\frac 1 2")).toParseLike("\\frac 1 2");
+    });
+});