diff --git a/lib/sinon/assert.js b/lib/sinon/assert.js
index 12d9676e2..bb25f5f6f 100644
--- a/lib/sinon/assert.js
+++ b/lib/sinon/assert.js
@@ -131,8 +131,14 @@ assert = {
     callCount: function assertCallCount(method, count) {
         verifyIsStub(method);
 
-        if (method.callCount !== count) {
-            var msg = "expected %n to be called " + timesInWords(count) +
+        var msg;
+        if (typeof count !== "number") {
+            msg =
+                `expected ${format(count)} to be a number ` +
+                `but was of type ${typeof count}`;
+            failAssertion(this, msg);
+        } else if (method.callCount !== count) {
+            msg = "expected %n to be called " + timesInWords(count) +
                 " but was called %c%C";
             failAssertion(this, method.printf(msg));
         } else {
diff --git a/test/assert-test.js b/test/assert-test.js
index edcf17a50..5b4137dfd 100644
--- a/test/assert-test.js
+++ b/test/assert-test.js
@@ -1345,6 +1345,18 @@ describe("assert", function () {
                           "once\n    doSomething()");
         });
 
+        it("assert.callCount exception message with non-numeric argument", function () {
+            this.obj.doSomething();
+
+            assert.equals(
+                this.message("callCount", this.obj.doSomething, "3").replace(
+                    / at.*/g,
+                    ""
+                ),
+                "expected '3' to be a number but was of type string"
+            );
+        });
+
         it("assert.calledOnce exception message", function () {
             this.obj.doSomething();
             this.obj.doSomething();