diff --git a/logics-js/logics.js b/logics-js/logics.js
index 58ce5ab..9548567 100644
--- a/logics-js/logics.js
+++ b/logics-js/logics.js
@@ -198,6 +198,7 @@ export default class Logics {
                     }
                 },
                 call: () => {
+                    let fname = node.children[0].match;
                     let args = [];
 
                     if (node.children.length === 2) {
@@ -205,7 +206,7 @@ export default class Logics {
                         args = stack.pop().toList();
                     }
 
-                    let fn = this.functions[node.children[0].match];
+                    let fn = this.functions[fname];
 
                     if (fn !== undefined) {
                         // Convert all args to Logics values
@@ -213,11 +214,15 @@ export default class Logics {
                             args[i] = new Value(args[i]);
                         }
 
-                        // todo: Handle invalid parameters
-                        stack.op0(fn(...args));
+                        try {
+                            stack.op0(fn(...args));
+                        }
+                        catch (e) {
+                            stack.op0(`#ERR:Invalid call to ${fname}()`)
+                        }
+
                     } else {
-                        // todo: should this return a string?
-                        throw new Error(`Call to unknown function: ${node.children[0].match}`);
+                        stack.op0(`#ERR:Call to unknown function ${fname}()`)
                     }
                 },
                 comprehension: () => {
diff --git a/logics-js/test/logics.js b/logics-js/test/logics.js
index a952075..004554d 100644
--- a/logics-js/test/logics.js
+++ b/logics-js/test/logics.js
@@ -23,7 +23,7 @@ function testcase(code) {
             switch (cmd[0].toLowerCase()) {
                 case "expect":
                     // Verify the expected result.
-                    let expect = cmd[1];
+                    let expect = cmd.slice(1).join(":");
 
                     // Ensure that the previous result is not undefined.
                     assert.notStrictEqual(lastResult, undefined);
diff --git a/logics-py/logics/logics.py b/logics-py/logics/logics.py
index 75c33f9..4dc7176 100644
--- a/logics-py/logics/logics.py
+++ b/logics-py/logics/logics.py
@@ -181,18 +181,22 @@ def _run(self, node: LogicsNode, stack: _Stack, values: dict):
                 return
 
             case "call":
+                fname = node.children[0].match
+
                 if len(node.children) > 1:
                     self._run(node.children[1], stack, values)
                     args = stack.pop().list()
                 else:
                     args = ()
 
-                if fn := self.functions.get(node.children[0].match):
-                    # todo: Handle invalid parameters
-                    stack.op0(fn(*args))
+                if fn := self.functions.get(fname):
+                    try:
+                        stack.op0(fn(*args))
+                    except TypeError:
+                        # TODO: Improve parameter validation
+                        stack.op0(f"#ERR:Invalid call to {fname}()")
                 else:
-                    # todo: should this return a string?
-                    raise NotImplementedError(f"Call to unknown function: {node.children[0].match!r}")
+                    stack.op0(f"#ERR:Call to unknown function {fname}()")
 
                 return
 
diff --git a/logics-py/tests/test_logics.py b/logics-py/tests/test_logics.py
index baf657b..f963197 100644
--- a/logics-py/tests/test_logics.py
+++ b/logics-py/tests/test_logics.py
@@ -19,7 +19,7 @@ def test_testcase(input):
 
             action = cmd[0].lower()
             if action == "expect":
-                expect = cmd[1]
+                expect = ":".join(cmd[1:])
 
                 assert last_result is not None, "#EXPECT used before evaluation"
 
diff --git a/tests/function_calls.lgx b/tests/function_calls.lgx
new file mode 100644
index 0000000..6d59e40
--- /dev/null
+++ b/tests/function_calls.lgx
@@ -0,0 +1,5 @@
+foo()
+#EXPECT:"#ERR:Call to unknown function foo()"
+
+upper()
+#EXPECT:"#ERR:Invalid call to upper()"