diff --git a/lib/sinon/assert.js b/lib/sinon/assert.js
index d2eb83335..da835b361 100644
--- a/lib/sinon/assert.js
+++ b/lib/sinon/assert.js
@@ -142,8 +142,14 @@ assert = {
     callCount: function assertCallCount(method, count) {
         verifyIsStub(method);
 
-        if (method.callCount !== count) {
-            var msg = "expected %n to be called " + timesInWords(count) + " but was called %c%C";
+        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 {
             assert.pass("callCount");
diff --git a/test/assert-test.js b/test/assert-test.js
index 596e090eb..e83b8c670 100644
--- a/test/assert-test.js
+++ b/test/assert-test.js
@@ -1427,6 +1427,18 @@ describe("assert", function() {
             );
         });
 
+        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();