diff --git a/lib/function/algebra/simplify/simplifyCore.js b/lib/function/algebra/simplify/simplifyCore.js
index 1ad03148f..9ccf7b2ff 100644
--- a/lib/function/algebra/simplify/simplifyCore.js
+++ b/lib/function/algebra/simplify/simplifyCore.js
@@ -123,15 +123,19 @@ function factory(type, config, load, typed, math) {
         }
         return new ParenthesisNode(c);
     } else if (node.isFunctionNode) {
-        var args = node.args.map(function (arg) {
-          return simplifyCore(arg)
-        });
-        if (args.length === 1) {
-            if (args[0].isParenthesisNode) {
-                args[0] = args[0].content;
+        if (node.fn == null) {
+            var args = node.args.map(function (arg) {
+              return simplifyCore(arg)
+            });
+            if (args.length === 1) {
+                if (args[0].isParenthesisNode) {
+                    args[0] = args[0].content;
+                }
             }
+            return new FunctionNode(node.name, args);
         }
-        return new FunctionNode(node.name, args);
+    } else {
+        // cannot simplify
     }
     return node;
   }
diff --git a/test/function/algebra/simplify.test.js b/test/function/algebra/simplify.test.js
index ceb405f24..3fe442f93 100644
--- a/test/function/algebra/simplify.test.js
+++ b/test/function/algebra/simplify.test.js
@@ -43,6 +43,17 @@ describe('simplify', function() {
     simplifyAndCompare('(-1)*x', '-x');
   });
 
+  it('should handle FunctionAssignmentNode', function() {
+    const node = math.expression.node;
+    const s = new node.FunctionAssignmentNode('sigma', ['x'], math.parse('1 / (1 + exp(-x))'));
+    const f = new node.FunctionNode(s, [new node.SymbolNode('x')]);
+    assert.equal(f.toString(), 'sigma(x) = 1 / (1 + exp(-x))(x)');
+    assert.equal(f.eval({x: 5}), 0.9933071490757153);
+    const fsimplified = math.simplify.simplifyCore(f);
+    assert.equal(fsimplified.toString(), 'sigma(x) = 1 / (1 + exp(-x))(x)');
+    assert.equal(fsimplified.eval({x: 5}), 0.9933071490757153);
+  });
+
   it('should simplify (n- -n1)', function() {
     simplifyAndCompare('2 + -3', '-1');
     simplifyAndCompare('2 - 3', '-1');